Drilling fluids and uses thereof

ABSTRACT

The present invention relates to drilling fluids which reduce fluid and cutting loss during the drilling of subterranean wells. More specifically, the drilling fluids disclosed herein comprise natural and synthetic polymer blends that are effective to provide the fluid with a high viscosity under low shear rates and a low viscosity under high shear rates. The present invention also relates to methods for using the drilling fluids for reducing fluid and cutting loss during drilling.

PRIORITY CLAIM

This is the US national phase of International Patent Application No.PCT/AU2019/050486, filed May 20, 2019, which claims priority toAustralian provisional patent application number 2018901763, filed May21, 2018, the entire contents of each of which are hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to drilling fluids which reducefluid and cutting loss during the drilling of subterranean wells. Morespecifically, the drilling fluids disclosed herein comprise natural andsynthetic polymer blends that are effective to provide the fluid with ahigh viscosity under low shear rates and a low viscosity under highshear rates.

BACKGROUND OF THE INVENTION

Mineral exploration and extraction require the drilling of subterraneanwells, also referred to as boreholes, from the surface of land underinvestigation. This is typically achieved through the use of rotatingdrill strings (being a series of attached, rigid, pipe sections) withdrill bits attached at one end to drill into the earth. As a rotatingdrill bit drills into the earth to form a borehole, additional pipesections are added in order to drill deeper, while the opposite occursas the drill bit is withdrawn from the borehole. Alternative drillingtechniques rely on coiled tubing instead of drill strings, wherein thecoiled tubing is generally of a ductile metal available in virtuallyunlimited lengths. In most coiled tubing drilling, a bottom holeassembly (BHA) located at the bottom of the tubing typically includes amud motor that powers and rotates a drill bit (given that the coiledtubing does not itself rotate), the mud motor being powered by themotion of drilling fluid pumped from the surface. In other forms ofcoiled tubing drilling, above-ground apparatus have been developed toallow for the rotation of the coiled tubing about its longitudinal axis.

Regardless of the technique employed, a drilling fluid (or mud) is usedto facilitate the drilling operation. Drilling fluids are speciallydesigned fluids that are circulated through a well as the borehole isbeing drilled. Drilling fluids serve a number of functions, includingbut not limited to promoting borehole stability, cooling and lubricatingthe drill bit and the drill string, assisting in the removal of drillcuttings from the well, aiding in support of the drill pipe and drillbit, transmitting hydraulic horsepower to a drilling motor, andstabilising and minimising fluid loss into a formation through which awell is being drilled.

An important property of the drilling fluid is its rheology, andspecific rheological parameters are intended for drilling andcirculating the fluid through the well. The fluid should be sufficientlyviscous to suspend drilled cuttings and to carry the cuttings to thewell surface. However, the fluid should not be so viscous as tointerfere with the drilling operation.

A common problem in drilling operations in mineral exploration, andpetroleum and geothermal drilling, is the loss of valuable drillingfluids and drill cuttings. For example, cuttings that encapsulateinformation about the mineralogy of the extracted rock can be lost intounconsolidated or fractured formations. Drilling fluid can also be lostinto fractures induced by excessive mud pressure, pre-existing openporosity/fractures, or large caverns in the formation. Indeed, fluidloss is a drilling challenge that can result in increases in tool wear,decreases in drilling rate, and can trigger borehole instabilityeventually leading to the complete loss of the well.

The conventional approach to control fluid and cutting loss is to uselost circulation materials to provide a physical barrier between theborehole and the permeable formations. Lost circulation materials suchas graded calcium carbonates, fibres and nutshells can be added to thedrilling fluid, which decrease the permeability and conductivity of theloss zone, and therefore results in fluid loss control. However,downhole motors typically used in drilling operations are oftensensitive to, and therefore can be damaged by, the presence of solidparticles in the drilling fluid. Furthermore, solid bridging agents mayplug pore throats in the reservoir rock. Finally, the effectiveness ofexisting drilling fluids is far from ideal with respect to fluid andcutting loss control during drilling.

Accordingly, there is a need for the formulation of borehole drillingfluids which have desired rheology and fluid and cutting loss controlproperties, including without the need to use solid bridging reagents.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention as it existed before the priority date of each claimof this application.

SUMMARY OF THE INVENTION

The present invention is predicated, in part, on the surprising findingthat a drilling fluid comprising particular combinations of natural andsynthetic polymers is capable of imparting superior fluid and cuttingloss control during borehole drilling when compared to a polymerdrilling fluid comprising the individual polymers or components inisolation, or compared to existing drilling fluids. The inventors havefound that the borehole drilling fluid of the present invention has aunique shear thinning property. It exhibits a low viscosity when exposedto high shear rates, for example when it drives downhole motors andcarries the cuttings in an annulus. However, the viscosity of thedrilling fluid increases significantly as the fluid penetrates intofractured and unconsolidated formations, where the fluid is under ordersof magnitude smaller shear rates. The increase in viscosity results inan ability of the drilling fluid to block the loss zone and improvedrilling fluid and cuttings recovery.

Accordingly, in a first aspect, the present invention provides aborehole drilling fluid comprising:

(i) xanthan gum;

(ii) low molecular weight partially-hydrolysed polyacrylamide (PHPA);and

(iii) low viscosity polyanionic cellulose (Pac-LV).

In a second aspect the present invention provides a method of reducingborehole drilling fluid loss and cutting loss during borehole drilling,the method comprising conducting the borehole drilling using a boreholedrilling fluid comprising:

(i) xanthan gum;

(ii) low molecular weight partially-hydrolysed polyacrylamide (PHPA);and

(iii) low viscosity polyanionic cellulose (Pac-LV).

In some embodiments the drilling fluid exhibits an increase in viscosityunder low shear rates. In some embodiments, when the shear rate of thedrilling fluid is less than about 0.01 1/s, the viscosity of thedrilling fluid is about 10000 cp or higher as measured at about 23° C.to about 25° C. In some embodiments, when the shear rate of the drillingfluid is about 0.01 1/s, the viscosity of the drilling fluid is about6,100 cp or higher as measured at about 23° C. to about 25° C.

In some embodiments the drilling fluid exhibits a decrease in viscosityunder high shear rates. In some embodiments, when the shear rate of thedrilling fluid is about 1000 1/s or more, the viscosity of the drillingfluid is about 12 cp or lower as measured at about 23° C. to about 25°C.

In some embodiments the drilling fluid comprises about 0.1% to about0.5% w/w xanthan gum, about 0.02% to about 0.1% w/w PHPA, and about0.02% to about 0.1% w/w Pac-LV. In some embodiments the drilling fluidcomprises about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, and about0.02% w/w Pac-LV.

In some embodiments the drilling fluid further comprises guar gum. Insome embodiments, the drilling fluid comprises up to about 0.1% w/w guargum. In some embodiments, the drilling fluid comprises about 0.02% w/wguar gum. In some embodiments the drilling fluid comprises about 0.18%w/w xanthan gum, about 0.05% w/w PHPA, about 0.02% w/w Pac-LV, and about0.02% guar gum.

In some embodiments the drilling fluid further comprises regularviscosity polyanionic cellulose (Pac-RV). In some embodiments, thedrilling fluid comprises up to about 0.1% w/w Pac-RV. In someembodiments, the drilling fluid comprises about 0.01% w/w Pac-RV. Insome embodiments, the drilling fluid comprises about 0.18% w/w xanthangum, about 0.05% w/w PHPA, about 0.02% w/w Pac-LV, about 0.02% w/w guargum, and about 0.01% w/w Pac-RV.

In some embodiments the drilling fluid further comprises potassiumchloride. In some embodiments, the drilling fluid comprises up to about8.0% w/w potassium chloride. In some embodiments, the drilling fluidcomprises about 4% potassium chloride. In some embodiments, the drillingfluid comprises about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, about0.02% w/w Pac-LV, about 0.02% w/w guar gum, about 0.01% w/w Pac-RV, andabout 4.0% w/w potassium chloride.

In some embodiments the drilling fluid is substantially free of solidparticles.

In some embodiments, the borehole drilling fluid of the aforementionedaspects of the present invention can comprise solid particles for use inparticular applications, such as when encountering significant fracturesduring drilling. In this regard, solid bridging agents can be added tothe drilling fluid to control fluid and cutting loss.

Accordingly, in some embodiments the drilling fluid comprises bentonite.In some embodiments, the drilling fluid comprises about 0.01% to about2.0% w/w bentonite. In some embodiments, the drilling fluid comprisesabout 1.2% w/w bentonite. In some embodiments, the drilling fluidcomprises about 0.3% w/w xanthan gum, about 0.05% w/w PHPA, about 0.02%w/w Pac-LV, and about 1.2% bentonite.

In some embodiments, the drilling fluid further comprises fibre. In someembodiments, the drilling fluid comprises up to about 5.0% w/w fibre. Insome embodiments, the drilling fluid comprises about 4.8% w/w fibre.

In some embodiments, the drilling fluid comprises about 0.3% w/w xanthangum, about 0.05% w/w PHPA, about 0.02% w/w Pac-LV, about 0.02% w/w guargum, about 0.01% w/w Pac-RV, about 1.2% w/w bentonite, and about 4.8%w/w fibre.

In some embodiments, the drilling fluid further comprises graphite. Insome embodiments, the drilling fluid comprises about 1.0% to about 10%w/w graphite. In some embodiments, the drilling fluid comprises about6.0% w/w graphite. In some embodiments, the drilling fluid comprisesabout 0.3% w/w xanthan gum, about 0.05% w/w PHPA, about 0.05% w/wPac-LV, about 0.02% w/w guar gum, about 1.2% bentonite, and about 6.0%w/w graphite.

BRIEF DESCRIPTION OF THE FIGURES

For a further understanding of the aspects and advantages of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying figures whichillustrate certain embodiments of the present invention.

FIG. 1—is a graph showing the variation of drilling fluid shear rate onfracture walls away from a borehole when the fluid loss is 100litres/minute passing through a 1 mm aperture fracture. The estimationsare for a fluid with a power index of 0.6.

FIG. 2—graphs of the results of rheology testing of an exemplarydrilling fluid using a HAAKE rheometer showing the variation in shearrate (FIG. 2A) and corresponding shear stress (FIG. 2B) placed on thefluid over time.

FIG. 3—graphs of the results of rheology testing of the exemplarydrilling fluid in FIG. 2 over a moderate range of shear rates. FIG. 3Apulls data from FIG. 2A in the moderate shear rate range (0 to 200 1/s).FIG. 3B shows the variation in shear stress placed on the fluid overthis moderate range of shear rates over time.

FIG. 4—graphs of the results of rheology testing of the exemplarydrilling fluid in FIG. 2 over a low range of shear rates. FIG. 4A pullsdata from FIG. 2A in the low shear rate range (0 to 0.1 1/s). FIG. 4Bshows the variation in shear stress placed on the fluid over this lowrange of shear rates over time.

FIG. 5—is a graph of the results of rheology testing (low shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.18% XG/0.05% PHPA/0.02% Pac-LV) compared to the individualcomponents of the drilling fluid alone.

FIG. 6—is a graph of the results of rheology testing (high shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.18% XG/0.05% PHPA/0.02% Pac-LV) compared to the individualcomponents of the drilling fluid alone.

FIG. 7—is a graph of the results of rheology testing (low shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.18% XG/0.05% PHPA/0.02% Pac-LV/0.02% GG) compared to theindividual components of the drilling fluid alone.

FIG. 8—is a graph of the results of rheology testing (low shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.18% XG/0.05% PHPA/0.02% Pac-LV/0.02% GG/0.01% Pac-RV)compared to the individual components of the drilling fluid alone.

FIG. 9—is a graph of the results of rheology testing (high shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.18% XG/0.05% PHPA/0.02% Pac-LV/0.02% GG/0.01% Pac-RV)compared to the individual components of the drilling fluid alone.

FIG. 10—is a graph of the results of rheology testing (low shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.3% XG/0.05% PHPA/0.02% Pac-LV/1.2% or 2.0% bentonite)compared to the individual components of the drilling fluid alone.

FIG. 11—is a graph of the results of rheology testing (high shear raterange) of a drilling fluid according to an embodiment of the presentinvention (0.3% XG/0.05% PHPA/0.02% Pac-LV/1.2% or 2.0% bentonite)compared to the individual components of the drilling fluid alone.

FIG. 12—is a graph of the results of rheology testing (low shear raterange) of drilling fluids according to embodiments of the presentinvention (0.18% XG/0.05% PHPA/0.02% Pac-LV or 0.18% XG/0.05% PHPA/0.02%Pac-LV/0.02% GG) compared to commercially available drilling fluids(Spectrocap, CR650 and Corewell).

FIG. 13—is a graph showing the performance of two drilling fluidsaccording to certain embodiments of the present invention (CTroI andCTroIX) in a first field trial compared to the performance of a drillingfluid composition comprising a single commercial polymer (Pac-RV) alone.

FIG. 14—is a graph tracking the progress of fluid loss during drillingin a second field trial.

FIG. 15—is a graph showing an improvement of fluid return during thesecond field trial drilling following injection of CTroIX duringdrilling.

FIG. 16—is a graph showing sustained drilling fluid return 25 minutesafter the loss zone was treated with CTroIX.

FIG. 17—is a graph showing the ability of a drilling fluid compositionaccording to an embodiment of the present invention (0.18% XG/0.05%PHPA/0.02% Pac-LV/0.02% GG/0.01% Pac-RV/4.0% KCl) to control drillingfluid loss during drilling unconsolidated formations in a third fieldtrial.

FIG. 18—is a graph of the results of the third field trial showingrepeated events of exposing virgin rock, complete drilling fluid lossoccurrence and resuming the return of fluid imparted by the drillingfluid composition according to FIG. 17 (data from FIG. 17).

FIG. 19—is a schematic of the Fluid Loss Simulator (FSL) used in Example3. A: transparent tube with sand and fluid; B: graduated receivingcylinder; C: air compressor; D: inlet pressure sensor (Wika); E: outletpressure sensor (1 Bar); F: DAQ; and G: computer with data acquisitionsoftware (CATMAN).

FIG. 20—is a graph showing a characteristic calibration plot of the FLSused in Example 3.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have determined that drilling fluid and cutting lossduring borehole drilling can be controlled by increasing the pressuredrop of drilling fluid flowing into fractures and unconsolidated ground.This can be achieved by extending the non-Newtonian behaviour of thedrilling fluid, where the fluid exhibits a high viscosity under lowshear rates and a low viscosity at high shear rates. Particular drillingfluid formulations/compositions have been identified by the inventor,which fulfil these properties.

Accordingly, certain disclosed embodiments provide compositions,methods, products, and uses thereof that have one or more advantages.For example, some of the advantages of some embodiments disclosed hereininclude one or more of the following: new drilling fluid compositionsfor borehole drilling; drilling fluid compositions that improve thecontrol of fluid and cutting loss during borehole drilling; drillingfluid compositions that are substantially free of solid particles andwhich are compatible with downhole motors used in drilling operations;new methods for borehole drilling which make use of the drilling fluidcompositions described herein; new methods for reducing fluid andcutting loss during borehole drilling; or the provision of a commercialalternative to existing drilling fluids and methods. Other advantages ofsome embodiments of the present disclosure are provided herein.

In one embodiment, the present invention provides a borehole drillingfluid comprising the following polymer components:

(i) xanthan gum;

(ii) low molecular weight partially-hydrolysed polyacrylamide (PHPA);and

(iii) low viscosity polyanionic cellulose (Pac-LV).

As used herein, the term “borehole” (and like terms such a “well” and“wellbore”) refers to a hole drilled into, or penetrating, the earth.This may include a hole drilled on land or on a seabed. A boreholeincludes any cased or uncased portions of the drilled well or any othertubulars in the drilled well. A borehole can have portions that arevertical, horizontal, or anything in between, and it can have portionsthat are straight, curved, or branched.

Xanthan gum (CAS Registry Number 111-38-66-2) is an extracellularpolymer produced by Gram negative bacteria of the genus Xanthomonas.Being a branched polysaccharide, it has a backbone of cellobiose as therepeating unit and side-chains consisting of a trisaccharide composed ofD-mannose (β-1,4), D-glucuronic acid (β-1,2) and D-mannose, which areattached to alternate glucose residues in the backbone by α-1,3linkages. The average composition of xanthan chains depends on theXanthomonas pathovar used and fermentation conditions. TypicalXanthomonas pathovars used to produce xanthan gum include X. arboricola,X. axonopodis, X. campestris, X. citri, X. fragaria, X. gummisudans, X.juglandis, X. phaseoli, X. vasculorium. However, Xanthomonas campestrisis the most common pathovar employed for industrial production ofxanthan gum.

The effect of production parameters, such as type of bioreactor,continuous or batch operation, type and concentration of nutrients inthe growth medium, optimum pH and temperature of growth medium andoxygen transfer rate, on the fermentation yield and xanthan molecularcharacteristics are well reported and well known to those skilled in theart.

As indicated above, the industrial production of xanthan is mainly basedon the fermentation of glucose using X. campestris. After thefermentation process the broth is pasteurized to eliminatemicroorganisms, xanthan is precipitated in alcohol, spray-dried, orre-suspended in water and precipitated. However, xanthan gum is readilycommercially available and can be obtained from sources such as generalglossary shops, pharmaceutical companies, and drilling fluidmanufacturers/distributors such as the Australian Mud Company (Balcatta,Western Australia, Australia), Baker Hughes (Houston, Tex., USA), M-ISwaco (USA), etc.

Xanthan gum is also known as Actigum CX 9, ADM 40, AMC XAN BORE, B 1459,Biopolymer 9702, Biopolymer XB 23, Biozan R, Bisfect XA 200, Bistop,Ceroga, Chemicogel, Dehydroxanthan gum, Duovis, E 415, Echogum, Echogum630, Echogum F, Echogum GM, Echogum RD, Echogum SF, Echogum T, Eco-Gum,Eco-Gum F, Ekogum ketorol, Enorflo X, Flocon 1035, Flocon 4800, Flocon4800C, Flodrill S, Galaxy XB, goma Xantham, Gomme xanthane, GUM,XANTHAN, Gumixan K, Gums, xanthomonas, Idvis, Inagel V 10, Inagel V 10K,Inagel V 7T, Jungbunzlauer ST, K 5C151, K 9C57, Kelco BT, Kelco CGT,Keldent, Kelflo, KELTROL, Keltrol 1000, Keltrol 630, Keltrol ASXT,Keltrol BT, Keltrol CG, Keltrol CGSFT, Keltrol CGT, Keltrol F, KeltrolHP, Keltrol K 5C151, Keltrol RD, Keltrol SF, Keltrol T, Keltrol TF,Keltrol TF 1000, Kelzan, Kelzan 140X, Kelzan AR, Kelzan ASX, KelzanASXT, Kelzan D, Kelzan F, Kelzan HP, Kelzan M, Kelzan MF, Kelzan RD,Kelzan S, Kelzan SS 4000, Kelzan ST, Kelzan T, Kelzan XC, Kelzan XCD,Kelzan XG, Kelzan ZN 4471116, Kem-Kh, Monad Gum DA, Monat Gum DA, MonatGum GS, Monategum GS, Neosoft XC, Neosoft XKK, Neosoft XO, Nomcort Z,Nomcort ZZ, Novaxan 200, N-VIS, OptiXan D, Orno X, PH Rapid,Polysaccharide B 1459, Polysaccharide gum, Rheoflow CD 1, Rheoflow CD 4,Rheogel, Rhodicare S, Rhodicare T, Rhodigel, Rhodigel 200, Rhodigel 23,Rhodigel 80, Rhodigel Clear, Rhodigel Ultra, Rhodoflood XR 75, Rhodopol23, Rhodopol 23P, Rhodopol 23U, Rhodopol 50MD, Rhodopol R 23, RhodopolT, Rhodopol XGD, Saboksan, San Ace, San Ace BS, San Ace C, San Ace E-S,San Ace NXG-C, San Ace NXG-S, Saraksan, Saraksan T, Satiaxane CX 90,Satiaxane CX 90T, Satiaxane CX 910, Satiaxane CX 911, ShelIflo XA,SHELLFLO XA 140, Soaxan, Soaxan XG 550, Statoil XC 44F4, TGCS, UltraXanthan, Ultra Xanthan V 7, Vanzan, Vanzan NF, VIS TOP D 3000, VIS TOP D3000C, VIS TOP D 3000DF-C, VS 900, VT 819, WT 5100, Xanbore, Xanflood,Xantham gum, Xanthan, Xanthan biopolymer, Xanthan Gum 614, Xanthan GumST, Xanthan Gummi, Xanthane gum, Xanthomonas gum, Xanthural 75, Xantural180, Xantural 75, Xanvis, XB 23, XC 8511-F4, XCD, XG 550, and X-VIS.

In some embodiments, the amount of xanthan gum present in the boreholedrilling fluid of the present invention may be in the range of about0.1% to about 0.5% by weight (w/w) of the drilling fluid, encompassingany value and range therebetween. For example, the xanthan gum may bepresent in a range of about 0.1% to 0.48%, 0.1% to 0.46%, 0.1% to 0.44%,0.1% to 0.42%, 0.1% to 0.4%, 0.1% to 0.38%, 0.1% to 0.36%, 0.1% to0.34%, 0.1% to 0.32%, 0.1% to 0.3%, 0.1% to 0.28%, 0.1% to 0.26%, 0.1%to 0.24%, 0.1% to 0.22%, 0.1% to 0.2%, 0.1% to 0.18%, 0.1% to 0.16%,0.1% to 0.14%, 0.1% to 0.12%, 0.12% to 0.5%, 0.12% to 0.48%, 0.12% to0.46%, 0.12% to 0.44%, 0.12% to 0.42%, 0.12% to 0.4%, 0.12% to 0.38%,0.12% to 0.36%, 0.12% to 0.34%, 0.12% to 0.32%, 0.12% to 0.3%, 0.12% to0.28%, 0.12% to 0.26%, 0.12% to 0.24%, 0.12% to 0.22%, 0.12% to 0.2%,0.12% to 0.18%, 0.12% to 0.16%, 0.12% to 0.14%, 0.14% to 0.5%, 0.14% to0.48%, 0.14% to 0.46%, 0.14% to 0.44%, 0.14% to 0.42%, 0.14% to 0.4%,0.14% to 0.38%, 0.14% to 0.36%, 0.14% to 0.34%, 0.14% to 0.32%, 0.14% to0.3%, 0.14% to 0.28%, 0.14% to 0.26%, 0.14% to 0.24%, 0.14% to 0.22%,0.14% to 0.2%, 0.14% to 0.18%, 0.14% to 0.16%, 0.16% to 0.5%, 0.16% to0.48%, 0.16% to 0.46%, 0.16% to 0.44%, 0.16% to 0.42%, 0.16% to 0.4%,0.16% to 0.38%, 0.16% to 0.36%, 0.16% to 0.34%, 0.16% to 0.32%, 0.16% to0.3%, 0.16% to 0.28%, 0.16% to 0.26%, 0.16% to 0.24%, 0.16% to 0.22%,0.16% to 0.2%, 0.16% to 0.18%, 0.18% to 0.5%, 0.18% to 0.48%, 0.18% to0.46%, 0.18% to 0.44%, 0.18% to 0.42%, 0.18% to 0.4%, 0.18% to 0.38%,0.18% to 0.36%, 0.18% to 0.34%, 0.18% to 0.32%, 0.18% to 0.3%, 0.18% to0.28%, 0.18% to 0.26%, 0.18% to 0.24%, 0.18% to 0.22%, 0.18% to 0.2%,0.2% to 0.5%, 0.2% to 0.48%, 0.2% to 0.46%, 0.2% to 0.44%, 0.2% to0.42%, 0.2% to 0.4%, 0.2% to 0.38%, 0.2% to 0.36%, 0.2% to 0.34%, 0.2%to 0.32%, 0.2% to 0.3%, 0.2% to 0.28%, 0.2% to 0.26%, 0.2% to 0.24%,0.2% to 0.22%, 0.22% to 0.5%, 0.22% to 0.48%, 0.22% to 0.46%, 0.22% to0.44%, 0.22% to 0.42%, 0.22% to 0.4%, 0.22% to 0.38%, 0.22% to 0.36%,0.22% to 0.34%, 0.22% to 0.32%, 0.22% to 0.5%, 0.22% to 0.48%, 0.22% to0.46%, 0.22% to 0.44%, 0.22% to 0.042%, 0.22% to 0.4%, 0.22% to 0.38%,0.22% to 0.36%, 0.22% to 0.34%, 0.22% to 0.32%, 0.22% to 0.3%, 0.22% to0.28%, 0.22% to 0.26%, 0.22% to 0.24%, 0.24% to 0.5%, 0.24% to 0.48%,0.24% to 0.46%, 0.24% to 0.44%, 0.24% to 0.42%, 0.24% to 0.4%, 0.24% to0.38%, 0.24% to 0.36%, 0.24% to 0.34%, 0.24% to 0.32%, 0.24% to 0.3%,0.24% to 0.28%, 0.24% to 0.26%, 0.26% to 0.5%, 0.26% to 0.48%, 0.26% to0.46%, 0.26% to 0.44%, 0.26% to 0.42%, 0.26% to 0.4%, 0.26% to 0.38%,0.26% to 0.36%, 0.26% to 0.34%, 0.26% to 0.32%, 0.26% to 0.3%, 0.26% to0.28%, 0.28% to 0.5%, 0.28% to 0.48%, 0.28% to 0.46%, 0.28% to 0.44%,0.28% to 0.42%, 0.28% to 0.4%, 0.28% to 0.38%, 0.28% to 0.36%, 0.28% to0.34%, 0.28% to 0.32%, 0.28% to 0.3%, 0.3% to 0.5%, 0.3% to 0.48%, 0.3%to 0.46%, 0.3% to 0.44%, 0.3% to 0.42%, 0.3% to 0.4%, 0.3% to 0.38%,0.3% to 0.36%, 0.3% to 0.34%, 0.3% to 0.32%, 0.32% to 0.5%, 0.32% to0.48%, 0.32% to 0.46%, 0.32% to 0.44%, 0.32% to 0.42%, 0.32% to 0.4%,0.32% to 0.38%, 0.32% to 0.36%, 0.32% to 0.34%, 0.34% to 0.5%, 0.34% to0.48%, 0.34% to 0.46%, 0.34% to 0.44%, 0.34% to 0.42%, 0.34% to 0.4%,0.34% to 0.38%, 0.34% to 0.36%, 0.36% to 0.5%, 0.36% to 0.48%, 0.36% to0.46%, 0.36% to 0.44%, 0.36% to 0.42%, 0.36% to 0.4%, 0.36% to 0.38%,0.38% to 0.5%, 0.38% to 0.48%, 0.38% to 0.46%, 0.38% to 0.44%, 0.38% to0.42%, 0.38% to 0.4%, 0.4% to 0.5%, 0.4% to 0.48%, 0.4% to 0.46%, 0.4%to 0.44%, 0.4% to 0.42%, 0.42% to 0.5%, 0.42% to 0.48%, 0.42% to 0.46%,0.42% to 0.44%, 0.44% to 0.5%, 0.44% to 0.48%, 0.44% to 0.46%, 0.46% to0.5%, 0.46% to 0.48%, and 0.48% to 0.5%, by w/w of the drilling fluid.

In some embodiments, the xanthan gum is present in an amount of up toabout 0.18% w/w of the drilling fluid. In some embodiments, the xanthangum is present in an amount of about 0.18% w/w of the drilling fluid.However, it would be appreciated by a person skilled in the art thathigher concentrations of xanthan gum (for example up to about 0.5%(w/w)) can be used at elevated temperatures such as those observed atdrilling depths beyond 500 metres.

As indicated above, low molecular weight partially-hydrolysedpolyacrylamide is another polymer component of the borehole drillingfluid of the present invention. Partially-hydrolysed polyacrylamide(also known as “PHPA”) is a synthetic polymer and can comprise polymersformed by polymerizing and subsequently hydrolyzing acrylamide (or alower homolog of acrylamide) or copolymerizing acrylamide with anacrylate, or the like. Such synthesis techniques are well known to aperson skilled in the art. When polyacrylamide is manufacturedcommercially, it normally contains 1 to 2 mole percent hydrolyzed(carboxylate) content that is inadvertently imparted during themanufacturing process. Indeed, polyacrylamide is normally not referredto as PHPA until the carboxylate content exceeds approximately 2 molepercent.

In some embodiments, the hydrolysis percentage of the PHPA (and/or theproportion of the amide groups of the polyacrylamide that are carboxylgroups or have been hydrolyzed to form carboxyl groups) may be in therange of a lower limit of about 5.0%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%,22.5%, and 25% to an upper limit of about 50%, 47.5%, 45%, 42.5%, 40%,37.5%, 35%, 32.5%, 30%, 27.5%, and 25%, encompassing any value andsubset therebetween. For example, in some embodiments, the hydrolysispercentage of the PHPA may be in the range of from about 20% to about40%, encompassing any value and subset therebetween.

The molecular weight of the PHPA may be in the range of a lower limit ofabout 10000 Da, 100000 Da, 1000000 Da, 2000000 Da, 3000000 Da, 4000000Da, 5000000 Da, 6000000 Da, 7000000 Da, 8000000 Da, 9000000 Da, and10000000 Da to an upper limit of about 22000000 Da, 21000000 Da,20000000 Da, 19000000 Da, 18000000 Da, 17000000 Da, 16000000 Da,15000000 Da, 14000000 Da, 13000000 Da, 12000000 Da, 11000000 Da, and10000000 Da, encompassing any value and subset therebetween. Forexample, in some embodiments, the molecular weight of the PHPA may be inthe range of from about 5000000 Da to about 22000000 Da, encompassingany value and subset therebetween.

The PHPA to be used in the drilling fluid of the present invention maybe in either an acid form or a salt form. In some embodiments, the PHPAis in a salt form, preferably a sodium salt form.

Examples of such PHPA polymers that are commercially available includepolymer distributors such as SNF Floerger (France), BASF (Houston, Tex.,USA), and also various drilling fluid suppliers such as Xinhai (China)and Beijing Hengju Chemical Group Corporation (China).

In some embodiments, the amount of PHPA present in the borehole drillingfluid of the present invention may be in the range of about 0.02% toabout 0.1% by weight (w/w) of the drilling fluid, encompassing any valueand range therebetween. For example, the PHPA may be present in a rangeof about 0.02% to 0.09%, 0.02% to 0.08%, 0.02% to 0.07%, 0.02% to 0.06%,0.02% to 0.05%, 0.02% to 0.04%, 0.02% to 0.03%, 0.03% to 0.1%, 0.03% to0.09%, 0.03% to 0.08%, 0.03% to 0.07%, 0.03% to 0.06%, 0.03% to 0.05%,0.03% to 0.04%, 0.04% to 0.1%, 0.04% to 0.09%, 0.04% to 0.08%, 0.04% to0.07%, 0.04% to 0.06%, 0.04% to 0.05%, 0.05% to 0.1%, 0.05% to 0.09%,0.05% to 0.08%, 0.05% to 0.07%, 0.05% to 0.06%, 0.06% to 0.1%, 0.06% to0.09%, 0.06% to 0.08%, 0.06% to 0.07%, 0.07% to 0.1%, 0.07% to 0.09%,0.07% to 0.08%, 0.08% to 0.1%, 0.08% to 0.09%, and 0.09% to 0.1% by w/wof the drilling fluid.

In some embodiments, the PHPA is present in an amount of up to about0.05% w/w of the drilling fluid. In some embodiments, the PHPA ispresent in an amount of about 0.05% w/w of the drilling fluid. However,it would be appreciated by a person skilled in the art thatconcentrations of PHPA higher and lower than this (and falling in therange of about 0.02% to about 0.1% w/w of the drilling fluid) may beused.

Low viscosity polyanionic cellulose (herein referred to as “Pac-LV”) isanother polymer component of the borehole drilling fluid of the presentinvention. Polyanionic cellulose (PAC) is a water-soluble celluloseether derivative made from natural cellulose by chemical modification.PAC is a white powder, is non-toxic, odorless, and is soluble in waterto form a viscous solution. PAC belongs to the polymer anionicelectrolytes, and is typically obtained from the isopropyl alcoholsolution of alkali cellulose and chloroacetic acid by an etherificationreaction. The raw materials for the production of PAC are similar tothose for the production of carboxymethyl cellulose, but in theproduction process, different degradation methods are employed so thatsubstitution of the hydroxyl group in the ring structure of theβ-glucose group is more uniform.

Pac-LV is also known as Pac-L and can be purchased from various drillingfluid suppliers such as the Australian Mud Company (Balcatta, WesternAustralia, Australia), Baker Hughes (Houston, Tex., USA), SidleyChemical (LinYi City, China), Global Drilling Fluids and Chemicals Ltd(Delhi, India) and Mud Logic (Australia).

In some embodiments, the amount of Pac-LV present in the boreholedrilling fluid of the present invention may be in the range of about0.02% to about 0.1% by weight (w/w) of the drilling fluid, encompassingany value and range therebetween. For example, the Pac-LV may be presentin a range of about 0.02% to 0.09%, 0.02% to 0.08%, 0.02% to 0.07%,0.02% to 0.06%, 0.02% to 0.05%, 0.02% to 0.04%, 0.02% to 0.03%, 0.03% to0.1%, 0.03% to 0.09%, 0.03% to 0.08%, 0.03% to 0.07%, 0.03% to 0.06%,0.03% to 0.05%, 0.03% to 0.04%, 0.04% to 0.1%, 0.04% to 0.09%, 0.04% to0.08%, 0.04% to 0.07%, 0.04% to 0.06%, 0.04% to 0.05%, 0.05% to 0.1%,0.05% to 0.09%, 0.05% to 0.08%, 0.05% to 0.07%, 0.05% to 0.06%, 0.06% to0.1%, 0.06% to 0.09%, 0.06% to 0.08%, 0.06% to 0.07%, 0.07% to 0.1%,0.07% to 0.09%, 0.07% to 0.08%, 0.08% to 0.1%, 0.08% to 0.09%, and 0.09%to 0.1% by w/w of the drilling fluid.

In some embodiments, the Pac-LV is present in an amount of up to about0.02% w/w of the drilling fluid. In some embodiments, the Pac-LV ispresent in an amount of about 0.02% w/w of the drilling fluid. However,it would be appreciated by a person skilled in the art thatconcentrations of Pac-Lv higher and lower than this (and falling in therange of about 0.02% to about 0.1% w/w of the drilling fluid) may beused.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, andabout 0.02% w/w Pac-LV.

In some embodiments, the drilling fluid of the present invention furthercomprises guar gum. Guar gum (CAS Registry Number 9000-30-0) is apolysaccharide composed of the sugars galactose and mannose. Thebackbone of guar gum is a linear chain of β 1,4-linked mannose residuesto which galactose residues are 1,6-linked at every second mannose,forming short side-branches.

Guar gum is made from guar beans which are principally grown in India,Pakistan, USA, Australia and Africa. The industrial production of guargum typically uses roasting, differential attrition, sieving, andpolishing processes. Guar seeds are separated from the plant and dried.Guar splits are soaked for prehydration, and then the soaked splits,which have reasonably high moisture content, are passed through aflaker. The flaked guar split is ground and then dried. Guar gum canalso be purchased from a number of commercial suppliers such as healthfood shops and chemists, Agro Gums (India), Global Drilling Fluids andChemicals Ltd (Delhi, India), and the Australian Mud Company (Balcatta,Western Australia, Australia).

Guar gum is also known as 1212A, Avicel CE 15, Burtonite V 7E, C 1000, C1000 (gum), C 250, C 250 (gum), Celbond 7, Celca-Gum D 49D, CG 70, CP3300, CSAA-M 80, CSA-M 175, Cyamopsis gum, Dealca TP 1, Dealca TP 2,Decorpa, Duck Gum 800, Dycol 4500, E 412, Edicol ULV 50, EGMB, EmcogumCSAA, Emulgum 200, Emulgum 200S, FFH 200, FG-HV, Fine Gum G, Fine Gum G17, Frimulsion BM, G 50, Galactasol, Galactasol 20H5FI, Galactasol 211,Galactasol 270, Galactasol 30M1F, Galaxy 1083, Gendril Thik, Gendriv162, goma guar, Gomme de guar, GR 10, Guapack PF 20, Guapack PN, Guar,Guar 5200, Guar flour, Guar gum (cyamopsis tetragonolobus), Guar Gummi,Guar HV 7000 CPS, Guar Supercol U Fine, Guar WW250F, Guaran, Guarcel302, Guarcol U 40, Guargel D 15, Gum cyamopsis, Gum guar, GUM, GUAR,Gums, guar, GV 23/2, GW 4, GW 4AFG, Herbapeck SF 08, Higum 551,HYDROXYPROPYL GALACTOMANNAN ETHER, Inagel GR 10, Inagel GR 10C, IndalcaAG, Indalca AG-BV, Indalca AG-HV, J 2Fp, J 3000, Jaguar 170, Jaguar2100, Jaguar 2204, Jaguar 2243, Jaguar 2513, Jaguar 2610, Jaguar 2638,Jaguar 387, Jaguar 6000, Jaguar 6003, Jaguar 6003VT, Jaguar 7500X,Jaguar 8200, Jaguar A 20B, Jaguar A 20D, Jaguar A 40F, Jaguar HP 140,Jaguar MDD, Jaguar MDD-I, K 4492, KWL 2000, Lameprint DX 9, Lamgum 200,Lej Guar, LGC 1, Lipocard, Loloss, Lycoid DR, Meyprofin M 175, Meyprogat30, Meyprogat 7, Meypro-Guar 50, Meypro-Guar CSAA 200/50, Meypro-GuarCSAA-M 225, Meyprogum L, Meyprogum TC 47, Neosoft G 11, Newgelin G 100,NGL 8158, Oruno G 1, PAK-T 80, Papsize 7, PF 20, Polytex 100, Procol F,Procol S 1, Procol U, Rantec 4000, Rantec D 1, Regonol, Rein Guarin, RG100, RT 3088, Soaguar RG 100, Solvent purified guar gum, Stamulcol ULV500, Super Tack, Supercol, Supercol G 2H, Supercol G 2S, Supercol GF,Supercol U, Supercol U Powder, Syngum D 46D, Uni-Guar, Uniguar 80,Vidocreme A, Vidogum G 120/1501, Vidogum G 200-1, Vidogum GH 175,Vidogum GHK 175, VIS TOP B 20, VIS TOP D 1321, VIS TOP D 20, VIS TOP D2022, VIS TOP D 2029, VIS TOP LH 303, Viscogum BCR 13/80, Viscogum HV100T, Viscogum HV 3000, Viscogum HV 3000A, VLV, WG 15, WG 19, WG 1 L,WOGU 4401, X 5363, α-D-Galactopyrano-β-D-mannopyranan, andα-D-galacto-β-D-Mannan.

The amount of guar gum present in the borehole drilling fluid accordingto an embodiment of the present invention may be up to about 0.1% byweight (w/w) of the drilling fluid. For example, the guar gum may bepresent in the range of about 0.01% to about 0.1% w/w of the drillingfluid, encompassing any value and range therebetween. For example, theguar gum may be present in a range of about 0.01% to 0.09%, 0.01% to0.08%, 0.01% to 0.07%, 0.01% to 0.06%, 0.01% to 0.05%, 0.01% to 0.04%,0.01% to 0.03%, 0.01% to 0.02%, 0.02% to 0.09%, 0.02% to 0.08%, 0.02% to0.07%, 0.02% to 0.06%, 0.02% to 0.05%, 0.02% to 0.04%, 0.02% to 0.03%,0.03% to 0.1%, 0.03% to 0.09%, 0.03% to 0.08%, 0.03% to 0.07%, 0.03% to0.06%, 0.03% to 0.05%, 0.03% to 0.04%, 0.04% to 0.1%, 0.04% to 0.09%,0.04% to 0.08%, 0.04% to 0.07%, 0.04% to 0.06%, 0.04% to 0.05%, 0.05% to0.1%, 0.05% to 0.09%, 0.05% to 0.08%, 0.05% to 0.07%, 0.05% to 0.06%,0.06% to 0.1%, 0.06% to 0.09%, 0.06% to 0.08%, 0.06% to 0.07%, 0.07% to0.1%, 0.07% to 0.09%, 0.07% to 0.08%, 0.08% to 0.1%, 0.08% to 0.09%, and0.09% to 0.1% by w/w of the drilling fluid.

In some embodiments, the guar gum is present in an amount of about 0.02%w/w of the drilling fluid. However, it would be appreciated by a personskilled in the art that concentrations of guar gum higher and lower thanthis (and present in an amount up to about 0.1% w/w of the drillingfluid) may be used.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.18% w/w xanthan gum, about 0.05% w/w/PHPA, about0.02% w/w Pac-LV, and about 0.02% guar gum.

In some embodiments, the drilling fluid of the present invention furthercomprises regular viscosity polyanionic cellulose (herein referred to as“Pac-RV”). Details regarding polyanionic cellulose are provided above.The difference between Pac-RV and Pac-LV is merely in the viscosityranges of the polyanionic cellulose. In this regard, Pac-RV providesmodifications at a wide range of shear rates, while Pac-LV mostlychanges the low end rheology and in other applications can help withfiltration of bentonite-base drilling fluids. Adding Pac-RV to waterresults in non-Newtonian behavior while adding Pac-LV increases theviscosity but mostly proportionally over the range of shear rates, i.e.fluid (Pac-LV and water) can be described as Newtonian. Pac-RV can bepurchased from the same sources as Pac-LV as indicated above.

The amount of Pac-RV present in the borehole drilling fluid according toan embodiment of the present invention may be up to about 0.1% by weight(w/w) of the drilling fluid. For example, the Pac-RV may be present inthe range of about 0.01% to about 0.1% w/w of the drilling fluid,encompassing any value and range therebetween. For example, the Pac-RVmay be present in a range of about 0.01% to 0.09%, 0.01% to 0.08%, 0.01%to 0.07%, 0.01% to 0.06%, 0.01% to 0.05%, 0.01% to 0.04%, 0.01% to0.03%, 0.01% to 0.02%, 0.02% to 0.09%, 0.02% to 0.08%, 0.02% to 0.07%,0.02% to 0.06%, 0.02% to 0.05%, 0.02% to 0.04%, 0.02% to 0.03%, 0.03% to0.1%, 0.03% to 0.09%, 0.03% to 0.08%, 0.03% to 0.07%, 0.03% to 0.06%,0.03% to 0.05%, 0.03% to 0.04%, 0.04% to 0.1%, 0.04% to 0.09%, 0.04% to0.08%, 0.04% to 0.07%, 0.04% to 0.06%, 0.04% to 0.05%, 0.05% to 0.1%,0.05% to 0.09%, 0.05% to 0.08%, 0.05% to 0.07%, 0.05% to 0.06%, 0.06% to0.1%, 0.06% to 0.09%, 0.06% to 0.08%, 0.06% to 0.07%, 0.07% to 0.1%,0.07% to 0.09%, 0.07% to 0.08%, 0.08% to 0.1%, 0.08% to 0.09%, and 0.09%to 0.1% by w/w of the drilling fluid.

In some embodiments, the Pac-RV is present in an amount of about 0.01%w/w of the drilling fluid. However, it would be appreciated by a personskilled in the art that concentrations of Pac-RV higher and lower thanthis (and present in an amount up to about 0.1% w/w of the drillingfluid) may be used.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.18% w/w xanthan gum, about 0.05% w/w/PHPA, about0.02% w/w Pac-LV, about 0.02% guar gum, and about 0.01% Pac-RV.

The aforementioned polymer components of the borehole drilling fluid ofthe present invention are contained in a base fluid, which may includeany suitable fluid known in the art, such as aqueous fluids, non-aqueousfluids, or any combination thereof, provided that the components arecompatible with the base fluid.

As an example, the base fluid may include an aqueous-based fluid, anaqueous-miscible fluid, a water-in-oil emulsion, an oil-in-wateremulsion, and an oil-based fluid. The base fluid can be obtained fromany source, provided that the fluid does not contain components thatadversely affect the stability and/or performance of the boreholedrilling fluid. Suitable fluid systems into which the principalcomponents of the borehole drilling fluid may be incorporated thereforeinclude water-based fluid systems, such as brines, and invert emulsionfluid systems.

Accordingly, in some embodiments, the borehole drilling fluid of thepresent invention is a water-based drilling fluid system, containing anaqueous base fluid. As used herein, “water-based” means that water or anaqueous solution is the dominant component of the drilling fluid (forexample, greater than 50% by weight of the drilling fluid). In thisregard, aqueous base fluids that are suitable may comprise water (fromany source). For example, the aqueous base fluid may include fresh wateror non-fresh water. Non-fresh water sources include surface water suchas brackish water, seawater, brine (e.g., saturated salt water),returned water (sometimes referred to as flowback water) from thedelivery of drilling fluid into a borehole, unused drilling fluid,produced water, salt water (e.g., water containing one or more saltsdissolved therein), or a combination thereof.

Where the aqueous base fluid comprises water with one or morewater-soluble salts dissolved therein, the one or more salts can mayinclude inorganic salts, formate salts, or a combination thereof.Examples of inorganic salts include monovalent salts (e.g. KCl, NaCl),alkali metal halides, and ammonium halides. Inorganic salts may alsoinclude divalent salts, such as alkaline earth metal halides (e.g.CaCl₂, CaBr₂, etc) and zinc halides. In some embodiments of the presentinvention, the borehole drilling fluid comprises potassium chloride(KCl). KCl can be added to improve the inhibition capacity of theborehole drilling fluid against shale formations that cause boreholeinstability.

The amount of KCl present in the borehole drilling fluid according to anembodiment of the present invention may be up to about 8.0% by weight(w/w) of the drilling fluid. For example, the KCl may be present in anamount up to about 7.5%, 7.0%, 6.5%, 6.0%, 5.5%, 5.0%, 4.5%, 4.0%, 3.5%,3.0%, 2.5%, 2.0%, 1.5%, 1.0%, 0.5%, down to about 0.1%. In someembodiments, the KCl is present in an amount of about 4.0% w/w of thedrilling fluid.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.18% w/w xanthan gum, about 0.05% w/w/PHPA, about0.02% w/w Pac-LV, about 0.02% guar gum, about 0.01% Pac-RV, and about4.0% KCl.

In some embodiments, the aqueous base fluid can comprise a monovalentbrine or a divalent brine. Suitable monovalent brines can include, forexample, sodium chloride brines, sodium bromide brines, potassiumchloride brines, potassium bromide brines, and the like. Suitabledivalent brines can include, for example, magnesium chloride brines,calcium chloride brines, calcium bromide brines, and the like.

In some embodiments, the density of the aqueous base fluid can beadjusted, among other purposes, to provide additional particulatetransport and suspension of the principal components included in theborehole drilling fluid. In some embodiments, the pH of the aqueous basefluid may also be adjusted (e.g., by using a buffer or other pHadjusting agent) to a specific level, which may depend on, among otherfactors, the principal components included in the borehole drillingfluid. One of ordinary skill in the art will understand when suchdensity and/or pH adjustments are appropriate. In some embodiments, thepH of the aqueous base fluid is in the range of about pH 8.0 to about pH11.0 and any range or value therebetween.

Suitable aqueous-miscible base fluids may include alcohols (e.g.,methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol,isobutanol, and t-butanol; glycerins); glycols (e.g., polyglycols,propylene glycol, and ethylene glycol); polyglycol amines; polyols; andany derivatives of the aforementioned. These may be included incombination with salts (e.g., sodium chloride, calcium chloride, calciumbromide, zinc bromide, potassium carbonate, sodium formate, potassiumformate, cesium formate, sodium acetate, potassium acetate, calciumacetate, ammonium acetate, ammonium chloride, ammonium bromide, sodiumnitrate, potassium nitrate, ammonium nitrate, ammonium sulfate, calciumnitrate, sodium carbonate, and potassium carbonate); and/or incombination with an aqueous-based fluid. Combinations of any of theaforementioned are also contemplated.

Suitable water-in-oil emulsions, also known as invert emulsions, mayhave an oil-to-water ratio from a lower limit of greater than about50:50, 55:45, 60:40, 65:35, 70:30, 75:25, or 80:20 to an upper limit ofless than about 100:0, 95:5, 90:10, 85:15, 80:20, 75:25, 70:30, or 65:35by volume in the base fluid, where the amount may range from any lowerlimit to any upper limit and encompassing any subset therebetween.

Examples of non-aqueous base fluids that may be suitable for use in theborehole drilling fluid of the present invention include, but are notlimited to, oils, hydrocarbons, organic liquids, alcohols, (e.g.,glycols), polar solvents, and the like. Suitable oil-based fluids mayinclude alkanes, olefins, aromatic organic compounds, cyclic alkanes,paraffins, diesel fluids, mineral oils, desulfurized hydrogenatedkerosenes, and any combination thereof.

In some embodiments, the borehole drilling fluid of the presentinvention may comprise a mixture of one or more fluids and/or gases,including but not limited to emulsions, foams, and the like. The basefluids for use in the present invention may additionally be gelled orfoamed by any means known in the art.

In some embodiments, the drilling fluid is substantially free of solidparticles. However, other conventional additives may be used in thefluid in combination with the aforementioned components. Examples ofsuch additional additives include, emulsifiers, acids, alkalinityagents, pH buffers, fluorides, fluid loss control additives, gases,nitrogen, carbon dioxide, surface modifying agents, tackifying agents,foamers, corrosion inhibitors, scale inhibitors, catalysts, clay controlagents, biocides, bactericides, friction reducers, antifoam agents,bridging agents, dispersants, flocculants, hhS scavengers, CO₂scavengers, oxygen scavengers, friction reducers, viscosifiers,breakers, relative permeability modifiers, resins, wetting agents,coating enhancement agents, filter cake removal agents, rheologymodifiers, filtration control agents, defoamers, surfactants, shalestabilizers, oils, and the like. One or more of these additives maycomprise degradable materials that are capable of undergoingirreversible degradation downhole. A person skilled in the art willunderstand the types of additives that may be included in the boreholedrilling fluid of the present invention.

In some embodiments, the term “substantially free of solid particles” istaken to mean no detectable levels of solid particles. However, the termalso encompasses drilling fluids wherein solid particles have beenremoved from the fluid using a 50 micron filter.

In some embodiments of the present invention, the borehole drillingfluid does not comprise a surfactant.

The aforementioned borehole drilling fluids of the present inventionexhibit advantageous rheology properties. Specifically, the drillingfluids exhibit an increase in viscosity under low shear rates, and adecrease in viscosity under high shear rates when compared to currentlyused drilling fluids and individual polymers used therein.

The viscosity of a fluid is its internal resistance to flow as measuredin centipoise (cp) units. The coefficient of viscosity of a normalhomogeneous fluid at a given temperature and pressure is a constant forthat fluid and independent of the rate of shear or the velocitygradient. Fluids that obey this rule are “Newtonian” fluids. In fluidscalled “non-Newtonian fluids,” this coefficient is not constant but is afunction of the rate at which the fluid is sheared as well as of therelative concentration of the phases. The drilling fluids of the presentinvention are generally non-Newtonian fluids. Non-Newtonian fluidsfrequently exhibit plastic flow, in which the flowing behavior of thematerial occurs after the applied stress reaches a critical value oryield point (YP). Yield points in drilling fluids are frequentlyexpressed in units of pounds per square 100 feet square, wherein theyield point is a function of the internal structure of the fluid. Indrilling, once the critical value or yield point (YP) of the drillingfluid is achieved, the rate of flow or rate of shear typically increaseswith an increase in pressure, causing flow or shearing stress. The rateof flow change, known as plastic viscosity (PV), is analogous toviscosity in Newtonian fluids and is similarly measured in centipoiseunits.

The relationship between the shear stress and shear rate can becharacterized by apparent viscosity. The apparent viscosity unlike theviscosity (for Newtonian Fluids) changes with shear rate. Differentmodels have been proposed to characterize the variation of apparentviscosity (or rather shear stress) with shear rate. Examples includeYield Bingham, Power Law, Yield Power Law and Casson models. While thecommon practice of industry is to use Bingham, the drilling simulatorsand research papers often tend to prefer to use the Yield Power Lawmodel.

The shear rate of a fluid is the rate at which the fluid is sheared or“worked” during flow. That is, it is the rate at which fluid layers orlaminae move past each other. Shear rate is determined by both thegeometry and speed of the flow of the fluid. For example, in the case ofa fluid flowing in a pipeline, the flow rate is related to the level ofshear rate and the pressure drop is correlated to the level of shearstress applied by the fluid on the pipe wall. The dimensions of shearrate is [1/T], e.g. 1/s, while Pa, lb/100 ft², or dial reading arecommonly used in quantifying shear stress in rheology studies.

There are different methods to measure fluid rheology such as capillary,hydraulic, flow-through constriction, oscillatory and rotary. In someembodiments, a rotary rheometer may be used. A rheometer is comprised ofan inner cylinder (called a bob) and an outer cylinder (called a rotor).The outer cylinder rotates while the torque applied by the fluid on theinner cylinder is measured. An advanced rheometer may also be used,which can provide viscosity measurements in a wide range of shear rates,for example from 0.001 1/s to 1900 1/s.

In some embodiments, when the shear rate of the drilling fluid of thepresent invention is less than about 0.01 1/s, the viscosity of thedrilling fluid is about 10000 cp or higher as measured at about 23° C.to about 25° C.

In some embodiments, when the shear rate of the drilling fluid of thepresent invention is about 0.01 1/s, the viscosity of the drilling fluidis about 6,100 cp or higher as measured at about 23° C. to about 25° C.

In some embodiments, when the shear rate of the drilling fluid of thepresent invention is about 1000 1/s or more, the viscosity of thedrilling fluid is about 12 cp or lower as measured at about 23° C. toabout 25° C.

In some embodiments, the borehole drilling fluid of the presentinvention can comprise solid particles for use in particularapplications, as described in further detail below. In this regard, oneor more bridging and/or weighting agents may be added to the drillingfluid.

Specifically, when encountering significant fractures during drilling,solid bridging agents can be added to the drilling fluid to controlfluid and cutting loss. In this regard, the usual approach to fluid-losscontrol in these circumstances is to substantially reduce thepermeability of the matrix of the fracture zone with a fluid-losscontrol material that blocks the permeability at or near the face of therock matrix of the fracture zone. For example, the fluid-loss controlmaterial may be a particulate that has a size selected to bridge andplug the pore throats of the matrix of the fracture. The higher theconcentration of the appropriately sized particulate, the fasterbridging will occur. As the fluid phase carrying the fluid-loss controlmaterial leaks into the fracture, the fluid-loss control materialbridges the pore throats of the matrix of the fracture and builds up onthe surface of the borehole or fracture face or penetrates only a littleinto the matrix. By accumulating solid particulate or other fluid-losscontrol material on the walls of a wellbore or a fracture, the fluidloss can be controlled. That is, the physical blockage of theconductivity of the fracture or unconsolidated formations by the lostcirculation material helps in controlling the fluid loss.

Fluid-loss control agents can include, for example, a filter cakeforming material, sometimes also known as a filtration control agent(such as clay (e.g., bentonite)) or an organic colloidal-sized solidparticulate (e.g., a biopolymer, cellulose polymer, or starch, modifiedstarch, plant tannin, a polyphosphate, a lignitic material, alignosulfonate, or a synthetic polymer), a filter cake bridging material(such as graphite, a calcium carbonate particulate, a celluloseparticulate, an asphalt particulate, and a gilsonite particulate), and alost circulation material to block larger openings in the formation(such as an appropriately-sized particulate of walnut shells, fibre, ormica, etc).

In some embodiments, the drilling fluid of the present inventioncomprises bentonite. Bentonite is an absorbent aluminium phyllosilicateclay consisting mostly of montmorillonite. Bentonite is mined from clayrich formations, and after crushing and drying can be packed. There aredifferent clay mines in the world producing different quality bentonite.Some of these bentonites might not have the required quality, and inthat case some additional additives are required before packing thebentonite. Examples of these additives are mix metal oxides or Starchpolymers. In some embodiments, raw bentonite (also known as API) can beused without any further additive.

Bentonite can be obtained from a number of commercial suppliers,including the Australian Mud Company (Balcatta, Western Australia,Australia), and Baker Hughes (Houston, Tex., USA).

The amount of bentonite present in the borehole drilling fluid accordingto an embodiment of the present invention may be up to about 2.0% byweight (w/w) of the drilling fluid. For example, the bentonite may bepresent in the range of about 0.01% to about 2.0% w/w of the drillingfluid, encompassing any value and range therebetween. For example, thebentonite may be present in a range of about 0.01% to 1.5%, 0.01% to1.0%, 0.01% to 0.5%, 0.01% to 0.1%, 0.01% to 0.05%, 0.05% to 2.0%, 0.05%to 1.5%, 0.05% to 1.0%, 0.05% to 0.5%, 0.05% to 0.1%, 0.1% to 2.0%, 0.1%to 1.5%, 0.1% to 1.0%, 0.1% to 0.5%, 0.5% to 2.0%, 0.5% to 1.5%, 0.5% to1.0%, 1.0% to 2.0%, 1.0% to 1.9%, 1.0% to 1.8%, 1.0% to 1.7%, 1.0% to1.6%, 1.0% to 1.5%, 1.0% to 1.4%, 1.0% to 1.3%, 1.0% to 1.2%, 1.0% to1.1%, 1.1% to 2.0%, 1.1% to 1.9%, 1.1% to 1.8%, 1.1% to 1.7%, 1.1% to1.6%, 1.1% to 1.5%, 1.1% to 1.4%, 1.1% to 1.3%, 1.1% to 1.2%, 1.2% to2.0%, 1.2% to 1.9%, 1.2% to 1.8%, 1.2% to 1.7%, 1.2% to 1.6%, 1.2% to1.5%, 1.2% to 1.4%, 1.2% to 1.3%, 1.3% to 2.0%, 1.3% to 1.9%, 1.3% to1.8%, 1.3% to 1.7%, 1.3% to 1.6%, 1.3% to 1.5%, 1.3% to 1.4%, 1.4% to2.0%, 1.4% to 1.9%, 1.4% to 1.8%, 1.4% to 1.7%, 1.4% to 1.6%, 1.4% to1.5%, 1.5% to 2.0%, 1.5% to 1.9%, 1.5% to 1.8%, 1.5% to 1.7%, 1.5% to1.6%, 1.6% to 2.0%, 1.6% to 1.9%, 1.6% to 1.8%, 1.6% to 1.7%, 1.7% to2.0%, 1.7% to 1.9%, 1.7% to 1.8%, 1.8% to 1.9%, and 1.9% to 2.0%, by w/wof the drilling fluid.

In some embodiments, the bentonite is present in an amount of about 1.2%w/w of the drilling fluid. However, it would be appreciated by a personskilled in the art that concentrations of bentonite higher or lower thanthis (and present in an amount up to about 2.0% w/w of the drillingfluid) may be used as dictated by the extent of the fracture andaccompanying fluid loss.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.3% w/w xanthan gum, about 0.05% w/w PHPA, about0.02% w/w Pac-LV, and about 1.2% w/w bentonite.

Fibre is another solid bridging agent that may be added to the drillingfluid of the present invention. The fibre can be synthetic or natural.The fibre does not change the fluid rheology of the base fluid, howeverit can block the aperture of fractured formations. Fibre can bepurchased from commercial sources such as the Australian Mud Company(Balcatta, Western Australia, Australia).

The amount of fibre present in the borehole drilling fluid according toan embodiment of the present invention may be up to about 5.0% by weight(w/w) of the drilling fluid. For example, the fibre may be present inthe range of about 0.1% to about 5.0% w/w of the drilling fluid,encompassing any value and range therebetween. For example, the fibremay be present in a range of about 0.1% to 4.8%, 0.1% to 4.6%, 0.1% to4.4%, 0.1% to 4.2%, 0.1% to 4.0%, 0.1% to 3.5%, 0.1% to 3.0%, 0.1% to2.5%, 0.1% to 2.0%, 0.1% to 1.5%, 0.1% to 1.0%, 0.1% to 0.5%, 0.5% to5.0%, 0.5% to 4.8%, 0.5% to 4.6%, 0.5% to 4.4%, 0.5% to 4.2%, 0.5% to4.0%, 0.5% to 3.5%, 0.5% to 3.0%, 0.5% to 2.5%, 0.5% to 2.0%, 0.5% to1.5%, 0.5% to 1.0%, 1.0% to 5.0%, 1.0% to 4.8%, 1.0% to 4.6%, 1.0% to4.4%, 1.0% to 4.2%, 1.0% to 4.0%, 1.0% to 3.5%, 1.0% to 3.0%, 1.0% to2.5%, 1.0% to 2.0%, 1.0% to 1.5%, 1.5% to 5.0%, 1.5% to 4.8%, 1.5% to4.6%, 1.5% to 4.4%, 1.5% to 4.2%, 1.5% to 4.0%, 1.5% to 3.5%, 1.5% to3.0%, 1.5% to 2.5%, 1.5% to 2.0%, 2.0% to 5.0%, 2.0% to 4.8%, 2.0% to4.6%, 2.0% to 4.4%, 2.0% to 4.2%, 2.0% to 4.0%, 2.0% to 3.5%, 2.0% to3.0%, 3.0% to 5.0%, 3.0% to 4.8%, 3.0% to 4.6%, 3.0% to 4.4%, 3.0% to4.2%, 3.0% to 4.0%, 3.0% to 3.5%, 4.0% to 5.0%, 4.0% to 4.8%, 4.0% to4.6%, 4.0% to 4.4%, 4.0% to 4.2%, 4.2% to 5.0%, 4.4% to 5.0%, 4.4% to5.0%, 4.8% to 5.0%, by w/w of the drilling fluid.

In some embodiments, the fibre is present in an amount of about 4.8% w/wof the drilling fluid. However, it would be appreciated by a personskilled in the art that concentrations of fibre higher or lower thanthis (and present in an amount up to about 5.0% w/w of the drillingfluid) may be used as dictated by the extent of the fracture andaccompanying fluid loss.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.3% w/w xanthan gum, about 0.02% w/w Pac-LV,about 0.02% w/w guar gum, about 0.01% w/w Pac-RV, about 1.2% w/wbentonite, and about 4.8% w/w fibre.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.3% w/w xanthan gum, about 0.05% w/w PHPA, about0.02% w/w Pac-LV, about 0.02% w/w guar gum, about 0.01% w/w Pac-RV,about 1.2% w/w bentonite, and about 4.8% w/w fibre.

As indicated above, graphite is another bridging material that may beused as a lost circulation material in the drilling fluid of the presentinvention. It has been demonstrated herein that the inclusion ofgraphite improves low end rheology of the drilling fluid and improvesthe seal/bridging capacity of the drilling fluid with respect tosubstantial fracture formations. The graphite is in powder form and canbe obtained from a number of commercial sources such as M-I Swaco(Houston, Tex., USA). In some embodiments, the median particle size(D50) of the graphite powder is in the range of 300 to 500 μm.

The amount of graphite present in the borehole drilling fluid accordingto an embodiment of the present invention may be up to about 10.0% byweight (w/w) of the drilling fluid. For example, the graphite may bepresent in the range of about 1.0% to about 10.0% w/w of the drillingfluid, encompassing any value and range therebetween. For example, thegraphite may be present in a range of about 1.0% to 9.0%, 1.0% to 8.0%,1.0% to 7.0%, 1.0% to 6.0%, 1.0% to 5.0%, 1.0% to 4.0%, 1.0% to 3.0%,1.0% to 2.0%, 2.0% to 10.0%, 2.0% to 9.0%, 2.0% to 8.0%, 2.0% to 7.0%,2.0% to 6.0%, 2.0% to 5.0%, 2.0% to 4.0%, 2.0% to 3.0%, 3.0% to 10.0%,3.0% to 9.0%, 3.0% to 8.0%, 3.0% to 7.0%, 3.0% to 6.0%, 3.0% to 5.0%,3.0% to 4.0%, 4.0% to 10.0%, 4.0% to 9.0%, 4.0% to 8.0%, 4.0% to 7.0%,4.0% to 6.0%, 4.0% to 5.0%, 5.0% to 10.0%, 5.0% to 9.0%, 5.0% to 8.0%,5.0% to 7.0%, 5.0% to 6.0%, 6.0% to 10.0%, 6.0% to 9.0%, 6.0% to 8.0%,6.0% to 7.0%, 7.0% to 10.0%, 7.0% to 9.0%, 7.0% to 8.0%, 8.0% to 10.0%,8.0% to 9.0%, and 9.0% to 10.0%, by w/w of the drilling fluid.

In some embodiments, the graphite is present in an amount of about 6.0%w/w of the drilling fluid. However, it would be appreciated by a personskilled in the art that concentrations of graphite higher or lower thanthis (and present in an amount up to about 10.0% w/w of the drillingfluid) may be used as dictated by the extent of the fracture andaccompanying fluid loss.

In some embodiments of the present invention, the borehole drillingfluid comprises about 0.3% w/w xanthan gum, about 0.05% w/w PHPA, about0.05% w/w Pac-LV, about 0.02% w/w guar gum, about 1.2% w/w bentonite,and about 6.0% w/w graphite.

Borehole drilling fluids according to the present invention may beprepared by any method suitable for a given application. For example,certain components of the borehole drilling fluid of embodiments of thepresent invention may be provided in a pre-blended liquid or powder or adispersion of powder in an aqueous or non-aqueous liquid, which may becombined with a base fluid at a subsequent time. After the pre-blendedliquids and the base fluid have been combined, other suitable additivesas described above may be added prior to introduction into the borehole.Those of ordinary skill in the art, with the benefit of this disclosurewill be able to determine other suitable methods for the preparation ofthe borehole drilling fluids of the present invention. For example, forlaboratory-based development, the components can be combined usingHamilton mixers which are API standard mixers. In the field, thecomponents can be added to water using hoppers, which utilize theventuri effect to introduce the components to a stream of water.

As indicated above, in some embodiments, the principal components of theborehole drilling fluid of the present invention include xanthan gum,PHPA and Pac-LV. This combination of polymers and additives has beenshown by the present inventors to exhibit surprising and advantageousrheology characteristics which enable a reduction in drilling fluid lossand cutting loss during borehole drilling when compared to existingdrilling fluids and when compared to the individual components of thedrilling fluid when used alone.

Accordingly, in a second aspect the present invention provides a methodof reducing borehole drilling fluid loss and cutting loss duringborehole drilling, the method comprising conducting the boreholedrilling using a borehole drilling fluid comprising:

(i) xanthan gum;

(ii) low molecular weight partially-hydrolysed polyacrylamide (PHPA);and

(iii) low viscosity polyanionic cellulose (Pac-LV).

These components, their source and the amounts useful are described indetail above. This combination of principal components may be used aloneor may be combined with one or more of guar gum, Pac-RV, and KCl.Accordingly, in some embodiments of the second aspect of the invention,the borehole drilling fluid also includes one or more of guar gum,Pac-RV, and KCl. These components, their source and the amounts usefulhave also been described above in detail.

In some embodiments of the second aspect of the present invention, thedrilling fluid is substantially free of solid particles.

However, as indicated above when encountering significant undergroundformations/fractures during drilling, solid bridging agents (lostcirculation materials) can be added to the drilling fluid to controlfluid and cutting loss.

Lost circulation involves the partial or complete loss of whole mud(both solid and continuous phase) to the underground formation Lostcirculation is one challenge that has plagued oil and gas drilling andexploration for decades. In fact, it has been argued that lostcirculation is one of the most cost inflating and time-consumingproblems faced by the oil and gas industry with an estimated annual costof over one billion dollars in rig time, materials (mud additives, etc.)and other resources. Lost circulation may occur naturally while drillingthrough highly permeable, cavernous formations, faults, and fissures, ormay be induced because of fractures created by excessive overbalance, ordrilling in a formation with a narrow mud weight window, or improperdrilling practices such as tripping too fast. Regardless of themechanism, lost circulation results in unwanted cost and non-productivetime during drilling. Therefore, the use of lost circulation materials(LCMs) as fluid loss additives can be used.

In this regard, in some embodiments of the second aspect of the presentinvention, the solid bridging reagents may include bentonite, fibre,and/or graphite. These components, their source and the amounts usefulhave also been described above in detail.

It is to be noted that where a range of values is expressed, it will beclearly understood that this range encompasses the upper and lowerlimits of the range, and all numerical values or sub-ranges in betweenthese limits as if each numerical value and sub-range is explicitlyrecited. For example, a range of “about 0.02% to about 0.1%”, or “about0.02% to 0.1%”, or like terminology, should be interpreted to includenot just about 0.02% to about 0.1%, but also the individual values(e.g., about 0.03%, about 0.04%, about 0.05%, about 0.06%, up to about0.1%) and the sub-ranges (e.g., about 0.03% to about 0.1%, about 0.04%to about 0.1%, about 0.05% to about 0.1%, etc) within the indicatedrange. The statement “about X % to Y %” has the same meaning as “about X% to about Y %,” unless indicated otherwise.

The term “about” as used in the specification means approximately ornearly and in the context of a numerical value or range set forth hereinis meant to encompass variations of +/−10% or less, +/−5% or less, +/−1%or less, or +/−0.1% or less of and from the numerical value or rangerecited or claimed.

As used herein, the singular forms “a,” “an,” and “the” may refer toplural articles unless specifically stated otherwise.

Throughout this specification, unless the context requires otherwise,the word “comprise”, or variations such as “comprises” or “comprising”,will be understood to imply the inclusion of a stated element or integeror group of elements or integers but not the exclusion of any otherelement or integer or group of elements or integers.

All methods described herein can be performed in any suitable orderunless indicated otherwise herein or clearly contradicted by context.The use of any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the exampleembodiments and does not pose a limitation on the scope of the claimedinvention unless otherwise claimed. No language in the specificationshould be construed as indicating any non-claimed element as essential.

It will be apparent to the person skilled in the art that while theinvention has been described in some detail for the purposes of clarityand understanding, various modifications and alterations to theembodiments and methods described herein may be made without departingfrom the scope of the inventive concept disclosed in this specification.

Furthermore, the description provided herein is in relation to severalembodiments which may share common characteristics and features. It isto be understood that one or more features of one embodiment may becombinable with one or more features of the other embodiments. Inaddition, a single feature or combination of features of the embodimentsmay constitute additional embodiments.

The subject headings used herein are included only for the ease ofreference of the reader and should not be used to limit the subjectmatter found throughout the disclosure or the claims. The subjectheadings should not be used in construing the scope of the claims or theclaim limitations.

The invention is further illustrated in the following examples. Theexamples are for the purpose of describing particular embodiments onlyand are not intended to be limiting with respect to the abovedescription. It will be appreciated by those skilled in the art that thedisclosure may be embodied in many other forms.

Example 1 Rheology Analysis of Drilling Fluid Formulations

The purpose of the present study was to use fluid rheology to identifydrilling fluid formulations useful for controlling fluid and cuttingloss during borehole drilling.

The common practice is to characterise drilling fluid properties usingan API rotary viscometer, for example. Using this machine, three mainmeasurements are recorded at rotary speeds of 3 rpm, 300 rpm and 600rpm, which correspond to shear rates of 5.1 1/s, 510 1/s and 1021 1/s,respectively. The latter two readings are typically used to characterisethe fluid response using rheological models such as the Bingham modeland power law models.

These measurements can provide a reasonable engineering estimation ofthe fluid behaviour for drilling fluid hydraulics and cuttingstransportation. However, the fluid is under a much smaller range ofshear rates when it is being lost into a formation. In order to show therelevant range of shear rates, here a horizontal fracture intersectingthe borehole with a fracture aperture of 1 mm is considered. Atheoretical model developed earlier for rectangular channels can be usedto predict the shear rate range ({dot over (γ)}_(w)) at various stagesof the fluid loss:

$\begin{matrix}{{{\overset{.}{\gamma}}_{w} = {{\overset{.}{\gamma}}_{a} \times \left( \frac{2}{3} \right)\left( {\frac{b^{*}}{f^{*}} + {\frac{a^{*}}{f^{*}}\frac{1}{n}}} \right)}}{{\overset{.}{\gamma}}_{a} = {\left( \frac{6q}{wh^{2}} \right)\left( {1 + \frac{h}{w}} \right) \times {f^{*}\left( \frac{H}{w} \right)}}}} & (23)\end{matrix}$

where q is the fluid loss into the fracture, h is the aperture, and w isthe width of the fracture. The parameter f* is related to geometry ofthe fracture and is dependent on the width and aperture ratios, and n isthe power index (Son Y, 2007, Polymer, 48(2): 632-637). Considering afluid loss of 100 lit/min into a 1 mm fracture, as shown in FIG. 1 thefluid shear rate evolves at various radii. In these equations, a* and b*are coefficients characterizing the geometry of a slit, and are afunction of the width and height of the fracture.

As is evident from FIG. 1, the drilling fluid is under a very smallrange of shear rates. Particularly, it is evident that after the fluidis a few meters away from the borehole, it is under shear rates lowerthan 0.05 1/s. This is in stark contrast to the shear rates encounteredby the drilling fluid during the drilling process.

In development of the drilling fluid formulations of the presentinvention, a rheometer was used (instead of a viscometer) so as to allowevaluation of the rheology of the drilling fluid formulations in a verybroad shear rate range (from 0.001 1/s to 1900 1/s), thereby cateringfor the actual changes in shear rates that may occur in the field duringdrilling and fracture encounters.

In this regard, a HAAKE Mars rheometer (from Thermo Fisher Scientific)was used in the testing of various drilling fluid formulations usingstandard methodology. The samples were prepared using the same procedurewith a 20 minute mixing time using Hamilton mixers. A rotary concentricsensor was used on the machine for most of the experiments, as it lendedto more consistent results. The temperature was always set at 25° C.,which was controlled using a water bath. Before placing the sample, themachine carries out a series of calibrations. Once the sample is placed,a shear rate sweep test was performed. The same procedure was used forall the samples. The duration of each shear rate was selected based ontrial and error. As the duration can be as long as 6 minutes for smallershear rates, the experiments were often conducted over a few hours. Thecollected data was then exported to excel files, which were thenanalyzed using a matlab code to measure the average of stabilized shearstress and viscosity at each shear rate.

Exemplary graphs showing the results of rheology testing on a polymersolution (0.2% w/w xanthan gum/0.03% w/w PHPA/0.03% w/w Pac-LV/0.02% w/wguar gum) are shown in FIGS. 2 to 4. These graphs show that for theexemplary fluid tested, the viscosity of a glycerin sample remainsrelatively constant over the tested range of shear rate, while thepolymer solution exhibits higher viscosities at low shear rates andlower viscosities at higher shear rates. These experiments were carriedout using an Ofite API rotary viscometer (Model 900 fitted with an R1B1head)(OFI Testing Equipment, Inc., Houston, Tex., USA).

The same rheology testing was applied to various fluid formulations thatwere prepared using combinations of synthetic and natural polymers, andother additives. The rheology of these formulations was compared to therheology of commercially available drilling fluids and to the rheologyof individual components of the fluid formulations being tested.Examples of fluids tested are as follows:

Xanthan Gum (XG) alone (0.1% to 0.5% w/w) —The desired amount of XanthanGum was slowly added to water under constant agitation. The rate ofaddition of the polymer was sufficiently slow to ensure proper mixingand to ensure that no fish eyes were formed. Xanthan Gum stock fromwhich desired dilutions were made was obtained from the Australian MudCompany (Balcatta, Western Australia, Australia).Partially-hydrolysed polyacrylamide (PHPA) alone (0.02% to 0.1% w/w)—The desired amount of PHPA was slowly added to water under constantagitation. The rate of addition of the polymer was sufficiently slow toensure proper mixing and to ensure that no fish eyes were formed. PHPAstock from which desired dilutions were made was obtained from a localdistributor (Canitis), with the original product supplied by Xinhai(China) under the name Hengfloc 6008.Low viscosity polyanionic cellulose (Pac-LV) alone (0.02% to 0.1% w/w)—The desired amount of Pac-LV was slowly added to water under constantagitation. The rate of addition of the polymer was sufficiently slow toensure proper mixing and to ensure that no fish eyes were formed. Pac-LVstock from which desired dilutions were made was obtained from theAustralian Mud Company or Mud Logic (Australia).Guar Gum (GG) alone (0.02% to 0.1% w/w) —The desired amount of guar gumpowder was slowly added to water under constant agitation. The rate ofaddition of the powder was sufficiently slow to ensure proper mixing andto ensure that no fish eyes were formed. Guar gum stock from whichdesired dilutions were made was obtained the Australian Mud Company orMud Logic (Australia).Bentonite alone (0.01% to 2.0% w/w) —The desired amount of bentonitepowder was slowly added to water under constant agitation. The rate ofaddition of the powder was sufficiently slow to ensure proper mixing andto ensure that no fish eyes were formed. Bentonite stock from whichdesired dilutions were made was obtained from the Australian Mud Company(Balcatta, Western Australia, Australia) and Baker Hughes (Houston,Tex., USA).Regular viscosity polyanionic cellulose (Pac-RV) alone (0.01% to 0.1%w/w) —The desired amount of Pac-RV was slowly added to water underconstant agitation. The rate of addition of the polymer was sufficientlyslow to ensure proper mixing and to ensure that no fish eyes wereformed. Pac-RV stock from which desired dilutions were made was obtainedthe Australian Mud Company.Xantham Gum (0.1% to 0.5% w/w)+PHPA (0.02% to 0.1% w/w)+Pac-LV (0.02% to0.1% w/w) —The desired amount of these three components was slowly addedto water under constant agitation. The rate of addition of the polymerswas sufficiently slow to ensure proper mixing and to ensure that no fisheyes were formed.Xantham Gum (0.1% to 0.5% w/w)+PHPA (0.02% to 0.1% w/w)+Pac-LV (0.02% to0.1% w/w)+Guar Gum (0.01% to 0.1% w/w) —The desired amount of these fourcomponents was slowly added to water under constant agitation. The rateof addition of the polymers was sufficiently slow to ensure propermixing and to ensure that no fish eyes were formed.Xantham Gum (0.1% to 0.5% w/w)+PHPA (0.02% to 0.1% w/w)+Pac-LV (0.02% to0.1% w/w)+Guar Gum (0.01% to 0.1% w/w)+Pac-RV (0.01% to 0.1% w/w) —Thedesired amount of these five components was slowly added to water underconstant agitation. The rate of addition of the polymers wassufficiently slow to ensure proper mixing and to ensure that no fisheyes were formed.Xantham Gum (0.1% to 0.5% w/w)+PHPA (0.02% to 0.1% w/w)+Pac-LV (0.02% to0.1% w/w)+Bentonite (0.01% to 2.0% w/w) —The desired amount of thesefour components was slowly added to water under constant agitation. Therate of addition of the components was sufficiently slow to ensureproper mixing and to ensure that no fish eyes were formed.

Corewell, Spectrocap and CR650

These three drilling fluids are commercially available and known.Corewell and Spectrocap are blends of polymers and CR650 is a PHPAdrilling polymer. All three products were purchased from a commercialsupplier (the Australian Mud Company, Balcatta, Australia). These threeproducts were tested at concentrations between 0.2-0.35% w/w until theprepared solutions had a similar high end viscosity at 1020 1/s(corresponding to 600 rpm).

Results of the rheology testing are shown in the tables below andaccompanying figures.

TABLE 1 Rheology testing of XanthanGum alone (0.2% w/w) - low endrheology t in s t_seg in s Á in 1/s , in Pa f in mPas T in ° C.Experiment 1 481.4369 151.5232 0.003 0.0315 10486.381 25.434 823.9627169.6154 0.004 0.0409 10204.246 25.432 1154.523 175.6455 0.005 0.04939850.970 25.440 1462.453 159.0599 0.006 0.0549 9144.264 25.440 1790.459159.8159 0.007 0.0616 8794.075 25.429 Experiment 2 496.8818 165.84840.003 0.03 9441.03 25.43 831.6734 175.6425 0.004 0.04 8864.80 25.431150.808 169.6143 0.005 0.04 7613.34 25.43 1505.523 199.0092 0.006 0.047128.33 25.43 1814.239 182.4298 0.007 0.05 6870.95 25.43 2127.6 170.42230.008 0.05 6620.05 25.43 2436.778 154.5387 0.009 0.06 6305.24 25.432794.263 187.7044 0.01 0.06 5624.53 25.43 3113.221 181.6721 0.02 0.094291.25 25.43 Experiment 3 875.181 157.5554 0.003 0.030516 10172.924.3502 1231.352 169.6152 0.004 0.037626 9406.893 24.34402 1568.455163.5796 0.005 0.043433 8687.255 24.33382 1918.699 170.3661 0.0059990.047331 7889.106 24.3215 2252.13 159.8179 0.007 0.051823 7403.77924.30519 2612.54 177.9512 0.007999 0.057685 7211.071 24.29726 2936.712160.6211 0.009 0.062118 6902.373 24.28915 3295.992 171.1211 0.010.064126 6412.763 24.27458 3597.15 128.2251 0.02 0.092396 4619.88224.25913 4042.2 229.9225 0.03 0.10485 3495.2 24.255 4378.645 222.37260.04 0.116145 2901.581 24.25081 Experiment 4 490.5468 158.3071 0.0030.022 7389.631 25.442 827.9073 170.3671 0.004 0.030 7500.708 25.4391171.431 188.4761 0.005 0.036 7137.284 25.436 1495.644 184.6967 0.0060.041 6785.400 25.432 1822.169 186.2629 0.007 0.045 6362.919 25.4332139.021 177.9095 0.008 0.049 6079.621 25.427 2443.784 157.5517 0.0090.051 5709.304 25.429 2761.514 150.0158 0.010 0.054 5363.171 25.4253085.142 148.5072 0.020 0.078 3910.088 25.430 Experiment 5 235.1571230.6667 0.003 0.036406 12482.47 25.43556 593 264.5789 0.004 0.04748910763.29 25.44132 868.4048 216.344 0.005 0.059504 9594.784 25.437861173.36 198.2554 0.006 0.067488 8734.823 25.4411 1505.057 206.5455 0.0070.112138 8068.282 25.43707 1812.352 189.9619 0.008 0.138133 7531.92725.44114 2161.75 214.8309 0.009 0.156426 7088.642 25.43809 2490.416220.1079 0.010 0.198 6714.38 25.43461 2769.027 174.9138 0.020 0.3276494698.508 25.43937 Average 0.003 9994.482 0.004 9347.987 0.005 8576.7270.006 7936.385 0.007 7500 0.008 6860.667 0.009 6501.39 0.010 6028.7120.020 4379.932 0.030 3495.2 0.040 2901.581 Note: t = time; s = seconds;Á in 1/s = shear rate; f in mPas = viscosity (cp); T = temperature

TABLE 2 Rheology testing of Xanthan Gum alone (0.2% w/w) - high endrheology Á in 1/s , in Pa f in mPas T in ° C. 100 5.95172 59.5175625.45049 200 8.006271 40.03146 25.45444 300 9.4824 31.60858 25.45033 40010.9991 27.49701 25.45063 500 12.84425 25.69 25.452 600 14.1094523.51567 25.45669 700 15.37769 21.968 25.45423 800 16.54439 20.6812325.45289 900 17.73983 19.71034 25.45538 1000 18.50234 18.50234 25.455841100 19.1495 17.40844 25.45469 1200 19.99728 16.66422 25.45116 130020.79256 15.99453 25.45427 1400 21.64892 15.46331 25.45701 1500 22.664815.1098 25.4521 1600 23.85268 14.90756 25.45827 1700 25.15163 14.7952325.45261 1800 26.35965 14.64358 25.44775 1900 27.64196 14.54843 25.4515

TABLE 3 Rheology testing of PHPA alone (0.25% w/w) t in s t_seg in s Áin 1/s , in Pa f in mPas T in ° C. 107.7706 101.5268 0.003 0.014254750.643 25.44318 352.5921 120.9838 0.005 0.025276 5055.539 25.44274573.1704 116.62 0.007 0.034687 4955.35 25.44337 813.9237 131.3046 0.010.040588 4058.73 25.44522 1019.827 113.0046 0.03 0.090315 3010.49225.44014 1254.35 122.6273 0.05 0.125148 2503.137 25.44601 1481.369124.1348 0.07 0.152777 2183.238 25.44547 1704.5 122.1268 0.1 0.1859561859.781 25.44294 1933.5 126.1448 0.3 0.371764 1239.141 25.44155 2163.18130.6688 0.5 0.474849 949.7043 25.44157 2366.37 108.5587 0.7 0.557626796.6484 25.44208 2585.405 102.5287 1 0.655151 655.1506 25.441451451.338 146.2434 3 0.988328 329.4421 25.44251 1815.698 186.2013 51.196513 239.3075 25.4479 2105.376 150.768 7 1.356107 193.7279 25.443222416.011 136.4466 10 1.540553 154.0563 25.44704 2761.387 156.7964 302.314417 77.14682 25.44316 3050.73 121.3896 50 2.822095 56.4424325.44319 3417.713 163.5846 70 3.241367 46.30508 25.44587 3742.533163.6068 100 3.77484 37.74888 25.44478 4094.961 188.4587 300 6.2797420.93255 25.44617 4400.885 169.6095 500 8.249778 16.4996 25.440214713.323 157.5546 700 9.99863 14.28381 25.44267 5010.411 129.6651 100012.56722 12.56734 25.44282 5355.619 150.0158 1300 18.16979 13.9769525.44224 5668.459 137.9547 1500 21.34214 14.2278 25.44098 6033.049177.9057 1700 24.68356 14.51982 25.44379 6324.326 143.9813 1900 28.1743114.82875 25.44117

TABLE 4 Rheology testing of Pac-LV alone (1.6% w/w) Á in 1/s SS, in Pa fin mPas T in ° C. 0.020007 0.010372 518.4099 25.73079 0.030009 0.014007466.7756 25.62659 0.040007 0.016334 408.2933 25.57672 0.050009 0.019019380.3237 25.5539 0.06001 0.021323 355.3221 25.54192 0.070008 0.021295304.1721 25.53106 0.080011 0.016075 200.907 25.52544 0.459763 0.01157423.64457 25.4672 0.668286 0.007743 10.23025 25.38908 0.849162 0.03311439.00379 25.35123 1 0.038164 38.16574 25.34311 2.626433 0.05356320.85174 25.33333 4 0.062769 15.69367 25.31333 5 0.074606 14.9223825.31946 5.999844 0.087957 14.66306 25.31014 6.945159 0.098164 14.1508825.29633 8.481502 0.095877 11.26925 25.30105 9.456285 0.108751 11.4820525.29609 14.61302 0.17788 11.85501 25.27487 24.1749 0.28362 11.5970325.27596 34.25049 0.428225 12.43685 25.26394 43.72733 0.51305 11.6838925.25783 60 0.731994 12.20012 25.26196 70 0.847187 12.10268 25.25358 800.97168 12.14701 25.24299 89.99991 1.091212 12.12434 25.24478 99.999941.224221 12.24233 25.23975 200 2.440153 12.20127 25.237 300 3.67334712.24493 25.22987 400 4.925526 12.31526 25.21442 499.9988 6.200512.40194 25.19525 599.9409 7.509364 12.51669 25.19221 699.9 8.83096312.61706 25.17423 799.9 10.18035 12.72682 25.16543 899.9 11.5776912.8651 25.15173 999.9 12.98921 12.9909 25.14644 1100 14.43265 13.1229125.1406 1200 15.90573 13.25667 25.14347 1300 17.41738 13.39953 25.159161400 18.96351 13.54608 25.16763 1500 20.54171 13.69547 25.1794 160022.13862 13.8377 25.17908 1700 23.775 13.98625 25.17646 1800 25.4359714.13229 25.17583 1900 27.06514 14.24557 25.18771 1900 27.07751 14.2520325.18802 2000 34.73916 17.3715 25.17178 2100 36.89519 17.5724 25.104132199.935 39.12565 17.78812 25.07266 2299.008 41.30244 17.96142 25.054722399 47.26972 19.70113 25.03423 2499 46.6228 18.65602 24.92247 259947.37625 18.22975 24.87633 2699 48.55631 17.99369 24.79274

TABLE 5 Rheology testing of 0.18% XG/0.05% PHPA/0.02% Pac-LV t in st_seg in s Á in 1/s , in Pa f in mPas T in ° C. 257.2791 241.22325580.001 0.014568 14572.09 24.55651 558.1933 206.5460674 0.002 0.02853114268.65 24.53449 881.49 192.2266667 0.003 0.041935 13979.56 24.503221219.795 192.2284091 0.004 0.056265 14067.84 24.47136 1532.676166.5985294 0.005 0.069336 13867.79 24.44015 1881.521 178.6570548 0.0060.081716 13619.66 24.40219 2213.193 173.3825185 0.007 0.087956 12565.4124.36874 2537.211 160.5670423 0.008 0.09228 11535.7 24.33845 2885.799171.8720863 0.009 0.100017 11115.4 24.31504 3213.194 162.0735484 0.010.106758 10671.45 24.28944 3537.953 150.015 0.02 0.1508 7540.64724.26327 3891.994 168.1083117 0.03 0.179974 5997.955 24.23532 4273.279211.8221154 0.04 0.197308 4932.212 24.20587 4575.615 176.39405590.049998 0.219587 4391.231 24.18622 4904.554 167.3490286 0.0599970.25108 4185.989 24.17211 5254.425 178.6574167 0.069999 0.2756923938.508 24.15908 5641.901 227.6549451 0.079998 0.275813 3447.60424.14099 5946.971 193.7321014 0.089998 0.304768 3386.37 24.123996272.598 180.3106557 0.099995 0.31727 3172.918 24.10926 6576.765146.9991912 0.2 0.433353 2166.912 24.09757 6926.864 159.0803704 0.30.505136 1683.778 24.08549 7297.4 189.9672414 0.4 0.562552 1406.46924.07207 7600.624 154.5349438 0.5 0.612534 1224.994 24.0623 7940.526156.04875 0.6 0.653 1088.365 24.05313 8281.61 159.0593023 0.7 0.686581980.8483 24.04256 8625.111 162.8308187 0.8 0.718585 898.2205 24.004158946.011 145.4926966 0.9 0.747393 830.4461 23.96742 9271.513 131.92456251 0.771613 771.585 23.94938 9635.086 156.8016 2 0.982154 491.088623.89423 9964.052 146.2451852 3 1.121007 373.6674 23.83326 10315.78159.8129814 4 1.225739 306.4447 23.80416 10645.65 150.7680745 5 1.312752262.5497 23.7787 10990.96 156.7979641 6 1.390814 231.803 23.7179611305.44 130.4188125 7 1.460288 208.6144 23.67938 11681.07 166.5971429 81.517007 189.6279 23.66321 12016.12 162.8238776 9 1.569374 174.378923.62293 12355.78 162.8247619 10 1.622401 162.2415 23.5685 9636.601158.3083916 2 0.982699 491.3545 23.89427 9976.87 159.0580137 3 1.120582373.5219 23.83171 10300.71 144.7368085 4 1.225851 306.4759 23.8046110641.14 146.2453293 5 1.31285 262.5677 23.77928 11005.31 171.119 61.390931 231.8238 23.71575 11306.19 131.1734694 7 1.460374 208.625223.67905 11681.07 166.5971429 8 1.517007 189.6279 23.66321 12010.84157.5475325 9 1.569409 174.3818 23.62383 12350.52 157.5485065 101.622357 162.237 23.56942 12645.45 111.7234821 20 2.026688 101.337523.54152 12850.24 97.70502994 30 2.316222 77.20832 23.53455 13075.48102.2216774 40 2.549497 63.73723 23.52213 13280.48 88.205 50 2.74454.87982 23.4897 13495.49 82.32736842 60 2.911564 48.52594 23.4481213735.49 104.0381203 70 3.066594 43.80752 23.41887 13940.48 84.5878554280 3.211596 40.14536 23.40843 14160.21 85.03937931 90 3.348386 37.2049723.40469 14390.53 94.99034921 100 3.473566 34.73577 23.38794 14605.4590.01429487 200 4.559083 22.79526 23.35583 14825.32 90.05602564 3005.400359 18.00109 23.32064 15040.12 85.04228144 400 6.179952 15.4504223.30419 15260.49 85.49278689 500 6.931344 13.86262 23.29672 15471.2774.63599229 600 7.66488 12.77452 23.29801 15710.49 93.63423611 7008.347063 11.92438 23.28104 15925.77 91.82448718 800 9.062776 11.328423.24885 16135.77 81.92211538 900 9.782295 10.86917 23.22218 16355.4982.3293985 1000 10.52128 10.52128 23.20714 16585.49 94.08443609 110011.28917 10.26286 23.20165 16795.45 83.69794872 1200 12.09808 10.0819223.20359 17040.59 107.7230693 1300 12.97723 9.982485 23.18911 17235.1182.78995506 1400 14.26961 10.19225 23.1591 17455.58 83.6825641 150016.09474 10.72987 23.13083 17690.57 99.51292683 1600 17.83577 11.1478923.11675 17895.3 83.79567164 1700 19.57164 11.51276 23.11149 18095.4964.23449624 1800 21.2515 11.80594 23.11226 18350.49 98.15423611 190022.93319 12.07042 23.11764

As can be seen from FIG. 5, the drilling fluid formulation of 0.18% w/wxanthan gum/0.05% w/w PHPA/0.02% Pac-LV demonstrated surprising superiorlow end rheology (increased viscosity under low shear rates) compared toxanthan gum, PHPA and Pac-LV alone. For example, at any given shearrate, the viscosity of the XG/PHPA/Pac-LV formulation was higher than afluid comprising XG, PHPA or Pac-LV alone. Specifically, when the shearrate of the XG/PHPA/Pac-LV formulation is less than about 0.01 1/s, theviscosity of the formulation is about 10000 cp or higher. Indeed, whilethe fluid comprising XG alone was the next best performing fluid withrespect to low end rheology, the viscosity of the fluid was only 6028 cpat a shear rate of 0.01 1/s. In contrast, the viscosity of theXG/PHPA/Pac-LV formulation was 10671 cp at a shear rate of 0.01 1/s.

Furthermore, as can be seen from FIG. 6, the high end rheology of theXG/PHPA/Pac-LV formulation was also surprisingly superior (i.e.decreased viscosity under high shear rates) compared to the individualcomponents of the drilling fluid alone. For example, at shear ratesbeyond 1000 1/s, the viscosity of the XG/PHPA/Pac-LV formulation waslower than a fluid comprising XG or Pac-LV alone. Specifically, when theshear rate of the XG/PHPA/Pac-LV formulation is about 1000 1/s or more,the viscosity of the formulation is about 12 cp or lower. Indeed, whilethe fluid comprising Pac-LV alone was the next best performing fluidwith respect to high end rheology, the viscosity of the fluid was notless than about 13 cp at shear rates above 1000 1/s.

Additional polymers were added to the core XG/PHPA/Pac-LV formulationand the results of the rheology testing are presented in Tables 6 to 9below, and in FIGS. 7 to 9.

TABLE 6 Rheology testing of Guar Gum alone (0.375% w/w) t in s t_seg ins Á in 1/s , in Pa f in mPas T in ° C. 299.7676 88.65826 0.003 0.002016672.1632 25.44973 513.5688 97.28365 0.004 0.002087 521.7447 25.44912703.291 81.99679 0.005 0.002175 435.1287 25.45258 919.4851 93.181490.006 0.002213 368.8731 25.4456 1126.535 94.99615 0.007 0.002268323.9925 25.44374 1330.661 94.08836 0.008 0.002357 294.7028 25.444241539.321 97.24978 0.009 0.002191 243.4842 25.44755 1742.644 95.443140.009999 0.001763 176.3521 25.45064 1948.59 96.79907 0.02 0.002763138.1464 25.44918 2146.774 90.47497 0.029999 0.004045 134.8264 25.443652354.055 96.80006 0.039999 0.004791 119.7775 25.44337 2556.24 94.084740.049998 0.005561 111.2287 25.44415 2761.989 94.55209 0.059997 0.007725128.7644 25.44697 2972.606 99.96644 0.069998 0.013272 189.6091 25.452883171.645 93.63157 0.079998 0.016831 210.394 25.45042 3571.244 82.778710.099995 0.014783 147.8396 25.44799 3775.954 82.32425 0.2 0.036457182.2935 25.44451 4002.293 103.5846 0.3 0.051676 172.26 25.455334192.349 88.6568 0.4 0.069199 173.0024 25.45337 4394.466 85.53905 0.50.085247 170.4932 25.45095 4615.028 100.4158 0.6 0.100448 167.421425.44903 4822.253 102.6799 0.7 0.114886 164.1313 25.44488 5017.54292.73056 0.8 0.130221 162.7705 25.448 5222.455 92.27659 0.9 0.144521160.5862 25.45533 5455.229 34.53922 2 0.297035 148.5018 25.448185539.458 33.14497 3.000006 0.417603 139.1933 25.44743 5628.273 36.9492 40.52774 131.942 25.4462 5713.538 36.26609 5.000006 0.629563 125.913325.44335 5795.683 32.99786 5.999994 0.725806 120.9628 25.45233 5881.64933.93407 7 0.816569 116.6494 25.44816 6050.279 31.50598 9 0.982658109.1863 25.45132 6140.643 36.94825 10 1.06076 106.076 25.45143 6223.57834.03928 20 1.709434 85.47168 25.45029 6305.122 30.4733 30 2.20813273.60462 25.45365 6397.096 36.56265 40 2.619288 65.48333 25.447566477.119 31.67395 50 2.982571 59.65164 25.44384 6566.639 34.83847 603.308657 55.1444 25.4459 6906.978 32.84885 100 4.391287 43.9132625.44773 6994.189 35.18037 200 6.249366 31.24671 25.44549 7082.65334.38684 300 7.667533 25.5588 25.44683 7169.744 36.0465 400 8.92413122.31031 25.446 7250.133 30.61667 500 10.08408 20.16781 25.451387339.766 34.68908 600 11.17042 18.6182 25.4512 7422.572 31.97644 70012.26102 17.51428 25.45203 7510.647 34.38786 800 13.33329 16.6671925.45299 7592.56 30.7679 900 14.38819 15.98689 25.44482 7681.74334.38684 1000 15.48174 15.48186 25.44485 7762.688 29.86416 1100 16.5654315.0594 25.44221 7854.154 35.74397 1200 17.66949 14.72462 25.44757941.253 37.2512 1300 18.76753 14.4366 25.4516 8022.164 32.60966 140019.95328 14.25268 25.44962 8112.201 34.63348 1500 21.12462 14.0834325.44396 8193.188 30.46523 1600 22.26807 13.91721 25.44802 8282.13133.63264 1700 23.46801 13.80528 25.45017 8366.148 31.97583 1800 24.6003313.66694 25.44503 8454.146 34.38813 1900 25.61398 13.48081 25.44935

TABLE 7 Rheology testing of 0.18% XG/0.05% PHPA/0.02% Pac-LV/0.02% GG tin s t_seg in s Á in 1/s , in Pa f in mPas T in ° C. 490.4221159.8121379 0.003 0.045 15075.793 25.437 822.3254 166.595 0.004 0.05714227.887 25.433 1148.429 168.1035714 0.005 0.068 13652.286 25.4301475.27 169.6130159 0.006 0.076 12735.873 25.438 1819.548 188.45414810.007 0.082 11681.481 25.438 2131.951 175.6446479 0.008 0.085 10607.67625.425 2453.924 172.6462879 0.009 0.087 9660.886 25.434 2779.025173.4107563 0.010 0.090 9014.420 25.438 3090.763 158.3066187 0.020 0.1286417.633 25.443

TABLE 8 Rheology testing of Pac-RV alone (0.275% w/w) t in s t_seg in sÁ in 1/s , in Pa f in mPas T in ° C. 307.3168 96.84746 0.003 −0.00353−1176.87 25.4473 515.4927 99.96404 0.004 −0.00256 −640.474 25.45298718.4363 97.70391 0.005 −0.00014 −27.0654 25.45006 921.9448 95.895520.006 0.003014 502.374 25.44624 1125.774 94.53877 0.007 0.0073751053.588 25.44621 1330.756 93.14878 0.008 0.005702 712.7273 25.44871537.388 95.45569 0.009 −0.00035 −39.2672 25.45559 1744.006 96.803230.009999 −1.28E−05 −1.27641 25.44784 1957.741 105.3908 0.02 0.003062153.1211 25.44657 2155.304 99.61646 0.029998 0.001839 61.30447 25.450372356.665 93.63511 0.04 0.006701 167.521 25.45182 2569.994 101.7750.049998 0.012804 256.0873 25.45024 2778.443 104.9386 0.059999 0.009522158.7107 25.44966 2977.393 98.60759 0.069998 0.003088 44.12083 25.449793176.882 92.72927 0.079998 0.00079 9.869708 25.45022 3587.439 92.729150.099994 0.008507 85.07768 25.44262 3792.305 92.27461 0.2 0.01018550.92509 25.44766 4002.631 97.24994 0.3 0.0191 63.66794 25.449 4204.98994.09583 0.4 0.023753 59.38326 25.44726 4413.298 96.79867 0.5 0.02868357.36541 25.45099 4620.989 99.05944 0.6 0.035334 58.89067 25.446834824.096 96.8012 0.7 0.040077 57.25311 25.44814 5028.166 95.89369 0.80.044889 56.11059 25.45449 5545.942 31.65523 3 0.152672 50.8970425.44799 5629.755 30.01477 4.000005 0.20252 50.6291 25.4488 5716.61531.20863 5 0.25051 50.10594 25.44656 5802.909 31.95433 6 0.29856149.75888 25.44807 5886.239 29.86617 7 0.344597 49.22876 25.452345975.418 33.44381 8 0.391249 48.90638 25.44763 6058.557 31.20766 90.437557 48.62104 25.45167 6142.701 29.86752 10 0.483657 48.3652225.45055 6233.518 35.0799 20 0.912163 45.60892 25.45578 6316.12 32.5789830 1.302967 43.43158 25.45322 6402.667 32.99556 40 1.661572 41.5388325.44883 6489.453 34.18814 50 1.991331 39.82698 25.44436 6574.83333.88996 60 2.295247 38.25362 25.44678 6663.096 36.56284 70 2.57868636.83942 25.44577 7002.669 34.23633 200 5.414783 27.07428 25.451087088.653 34.03811 300 7.100977 23.67006 25.44948 7175.994 35.68743 4008.590309 21.47562 25.44877 7257.897 30.76142 500 9.973277 19.9466225.45282 7343.279 30.46171 600 11.28949 18.81604 25.44919 7430.14331.65476 700.0005 12.54799 17.92577 25.45079 7516.478 32.40105 80013.76973 17.21136 25.45168 7601.141 31.82328 900 14.96353 16.6266325.45397 7691.179 35.67631 1000 16.14019 16.14025 25.44926 7775.9434.78183 1100 17.29935 15.72702 25.44815 7857.694 31.22196 1200 18.4426915.36892 25.45226 7948.693 36.49739 1300 19.57353 15.05712 25.447718031.598 33.14674 1400 20.69469 14.78106 25.45156 8119.538 35.74481 150021.80744 14.53925 25.44919 8200.158 30.61753 1600 22.90684 14.3179125.45133 8292.35 36.41581 1700 24.00229 14.11968 25.45229 8372.10430.77018 1800 25.09366 13.94049 25.44738 8450.471 23.22958 1900 26.1807113.77961 25.44948

TABLE 9 Rheology testing of 0.18% XG/0.05% PHPA/0.02% Pac-LV/0.02%GG/0.01% Pac-RV t in s t_seg in s Á in 1/s , in Pa f in mPas T in ° C.257.2791 241.2233 0.001 0.014568 14572.09 24.55651 558.1933 206.54610.002 0.028531 14268.65 24.53449 881.49 192.2267 0.003 0.041935 13979.5624.50322 1219.795 192.2284 0.004 0.056265 14067.84 24.47136 1532.676166.5985 0.005 0.069336 13867.79 24.44015 1881.521 178.6571 0.0060.081716 13619.66 24.40219 2213.193 173.3825 0.007 0.087956 12565.4124.36874 2537.211 160.567 0.008 0.09228 11535.7 24.33845 2885.799171.8721 0.009 0.100017 11115.4 24.31504 3213.194 162.0735 0.01 0.10675810671.45 24.28944 3537.953 150.015 0.02 0.1508 7540.647 24.263273891.994 168.1083 0.03 0.179974 5997.955 24.23532 4273.279 211.8221 0.040.197308 4932.212 24.20587 4575.615 176.3941 0.049998 0.219587 4391.23124.18622 4904.554 167.349 0.059997 0.25108 4185.989 24.17211 5254.425178.6574 0.069999 0.275692 3938.508 24.15908 5641.901 227.6549 0.0799980.275813 3447.604 24.14099 5946.971 193.7321 0.089998 0.304768 3386.3724.12399 6272.598 180.3107 0.099995 0.31727 3172.918 24.10926 6576.765146.9992 0.2 0.433353 2166.912 24.09757 6926.864 159.0804 0.3 0.5051361683.778 24.08549 7297.4 189.9672 0.4 0.562552 1406.469 24.072077600.624 154.5349 0.5 0.612534 1224.994 24.0623 7940.526 156.0488 0.60.653 1088.365 24.05313 8281.61 159.0593 0.7 0.686581 980.8483 24.042568625.111 162.8308 0.8 0.718585 898.2205 24.00415 8946.011 145.4927 0.90.747393 830.4461 23.96742 9271.513 131.9246 1 0.771613 771.585 23.949389635.086 156.8016 2 0.982154 491.0886 23.89423 9964.052 146.2452 31.121007 373.6674 23.83326 10315.78 159.813 4 1.225739 306.4447 23.8041610645.65 150.7681 5 1.312752 262.5497 23.7787 10990.96 156.798 61.390814 231.803 23.71796 11305.44 130.4188 7 1.460288 208.6144 23.6793811681.07 166.5971 8 1.517007 189.6279 23.66321 12016.12 162.8239 91.569374 174.3789 23.62293 12355.78 162.8248 10 1.622401 162.241523.5685 9636.601 158.3084 2 0.982699 491.3545 23.89427 9976.87 159.058 31.120582 373.5219 23.83171 10300.71 144.7368 4 1.225851 306.475923.80461 10641.14 146.2453 5 1.31285 262.5677 23.77928 11005.31 171.1196 1.390931 231.8238 23.71575 11306.19 131.1735 7 1.460374 208.625223.67905 11681.07 166.5971 8 1.517007 189.6279 23.66321 12010.84157.5475 9 1.569409 174.3818 23.62383 12350.52 157.5485 10 1.622357162.237 23.56942 12645.45 111.7235 20 2.026688 101.3375 23.5415212850.24 97.70503 30 2.316222 77.20832 23.53455 13075.48 102.2217 402.549497 63.73723 23.52213 13280.48 88.205 50 2.744 54.87982 23.489713495.49 82.32737 60 2.911564 48.52594 23.44812 13735.49 104.0381 703.066594 43.80752 23.41887 13940.48 84.58786 80 3.211596 40.1453623.40843 14160.21 85.03938 90 3.348386 37.20497 23.40469 14390.5394.99035 100 3.473566 34.73577 23.38794 14605.45 90.01429 200 4.55908322.79526 23.35583 14825.32 90.05603 300 5.400359 18.00109 23.3206415040.12 85.04228 400 6.179952 15.45042 23.30419 15260.49 85.49279 5006.931344 13.86262 23.29672 15471.27 74.63599 600 7.66488 12.7745223.29801 15710.49 93.63424 700 8.347063 11.92438 23.28104 15925.7791.82449 800 9.062776 11.3284 23.24885 16135.77 81.92212 900 9.78229510.86917 23.22218 16355.49 82.3294 1000 10.52128 10.52128 23.2071416585.49 94.08444 1100 11.28917 10.26286 23.20165 16795.45 83.69795 120012.09808 10.08192 23.20359 17040.59 107.7231 1300 12.97723 9.98248523.18911 17235.11 82.78996 1400 14.26961 10.19225 23.1591 17455.5883.68256 1500 16.09474 10.72987 23.13083 17690.57 99.51293 1600 17.8357711.14789 23.11675 17895.3 83.79567 1700 19.57164 11.51276 23.1114918095.49 64.2345 1800 21.2515 11.80594 23.11226 18350.49 98.15424 190022.93319 12.07042 23.11764

As can be seen from FIG. 7, the XG/PHPA/Pac-LV/GG formulation maintainedsuperior low end rheology compared to xanthan gum, guar gum and Pac-LValone. That is, at any given low end shear rate, the viscosity of theXG/PHPA/Pac-LV/GG formulation was higher than a fluid comprising XG, GG,or Pac-LV alone.

As can be seen from FIG. 8, the XG/PHPA/Pac-LV/GG/Pac-RV formulationalso maintained superior low end rheology compared to xanthan gum, guargum, Pac-LV, and Pac-RV alone. Specifically, at any given low end shearrate, the viscosity of the XG/PHPA/Pac-LV/GG/Pac-RV formulation washigher than a fluid comprising individual components alone. FIG. 9 showsthe results of rheology testing of the XG/PHPA/Pac-LV/GG/Pac-RVformulation at high shear rates. As can be seen from the Figure, theXG/PHPA/Pac-LV/GG/Pac-RV formulation also maintained superior high endrheology compared to a fluid comprising xanthan gum, guar gum, Pac-LV,Pac-RV alone or PHPA alone. Indeed, other than theXG/PHPA/Pac-LV/GG/Pac-RV formulation, none of the fluids containingindividual components alone could achieve a viscosity of about 12 cp orlower at shear rates of about 1000 1/s or more.

These results establish that a drilling fluid formulation comprising atleast the minimal xanthan gum/PHPA/Pac-LV polymers imparts an unexpectedand superior low end and high end rheology to the fluid.

To test whether this minimal polymer combination could also be usefulfor controlling fluid and cutting loss when encountering fractureformations during drilling, the particulate bentonite was added andfurther rheology testing conducted. The results are shown in Tables 10to 12 and in FIGS. 10 and 11.

TABLE 10 Rheology testing of Bentonite alone (3.0% w/w) t in s t_seg ins Á in 1/s , in Pa f in mPas T in ° C. 2759.667 88.65638 0.0599990.302202 5036.596 25.4459 2976.018 100.4159 0.069999 0.258874 3698.35325.45198 3176.5 96.34338 0.079998 0.227519 2844.065 25.45019 3389.741103.4935 0.089994 0.229294 2547.494 25.44753 3591.528 100.8685 0.0999930.241364 2413.977 25.45023 3787.88 92.27296 0.2 0.218864 1094.112 25.4483993.046 92.27798 0.3 0.226243 754.2156 25.44809 4214.994 109.007 0.40.219184 547.9867 25.44823 4411.48 100.4166 0.5 0.210305 420.519225.45299 4616.278 100.4176 0.6 0.204479 340.855 25.45225 4819.58198.61782 0.7 0.199788 285.4218 25.45402 5025.24 99.51061 0.8 0.193246241.5475 25.45352 5064.14 138.4046 0.8 0.191452 239.3151 25.453665229.309 98.60707 0.9 0.18842 209.3602 25.4521 5370.094 34.23525 10.182482 182.4776 25.45494 5450.78 30.01381 2 0.228145 114.069 25.450755543.1 37.25047 3 0.262053 87.34493 25.4498 5630.338 38.68289 3.9999930.291118 72.7736 25.45154 5712.333 33.44292 5 0.31122 62.24362 25.454245800.712 37.00773 6 0.330791 55.1285 25.45098 5881.728 32.99755 7.0000110.344422 49.20672 25.44794 5969.594 36.04563 8 0.365438 45.6760625.44844 6054.988 35.8212 9 0.399919 44.42845 25.45261 6136.152 32.2763910 0.456825 45.68216 25.45462 6309.098 34.68687 30 3.282656 109.41925.45172 6396.608 36.1968 40 3.239843 80.99667 25.44908 6478.21332.57903 50 3.170579 63.41306 25.4471 6567.261 36.49662 60 3.16438952.73943 25.45083 6650.146 34.08522 70 3.18752 45.53608 25.4483 6736.65934.93996 80 3.191096 39.88892 25.45509 6821.159 34.53729 90 3.21398835.71141 25.44947 6903.068 31.22054 100 3.312679 33.12679 25.443116989.2 31.50562 200 4.462711 22.31347 25.44937 7081.277 38.90861 3005.317292 17.72467 25.44737 7162.189 34.84067 400 5.957622 14.8942125.44915 7251.593 38.75979 500 6.62235 13.24493 25.44779 7333.73835.52513 600 7.45625 12.42725 25.45038 7417.678 33.32997 700 8.35283911.93293 25.45144 7501.068 31.25751 800 9.001768 11.25258 25.448267586.068 30.92012 900 9.652579 10.72479 25.44705 7670.061 29.56285 100010.31995 10.31995 25.44655 7761.153 35.29178 1100 10.99288 9.99376125.44865 7843.207 32.12546 1200 11.73755 9.781315 25.44337 7927.21830.46182 1300 12.5203 9.63102 25.4431 8017.091 35.14006 1400 13.374889.553421 25.44713 8097.184 29.71315 1500 14.74694 9.831209 25.447148187.551 34.6868 1600 16.40993 10.25612 25.45279 8270.19 31.82415 170018.23571 10.72668 25.44364 8361.653 38.00374 1800 19.96476 11.0914325.44898 8439.16 29.8642 1900 21.62679 11.38191 25.44221

TABLE 11 Rheology testing of 0.3% XG/ 0.05% PHPA/0.02% Pac-LV/1.2%Bentonite t in s t_seg in s Á in 1/s , in Pa f in mPas 497.298 166.59770.005 0.52 104104.41 883.2299 229.1649 0.007 0.67 95278.44 1161.105183.1783 0.010 0.79 79045.70 1473.262 171.8743 0.030 1.18 39424.541794.57 168.8575 0.050 1.40 27925.04 2116.492 166.5975 0.070 1.5321918.17 2455.202 180.9342 0.100 1.67 16653.76 2724.159 126.022 0.5002.50 5002.01 3085.539 161.3157 1 2.85 2850.78 3416.076 168.1712 2 3.271635.80 3764.935 192.9804 5 3.94 788.67 4068.624 173.4179 10 4.56 456.064413.222 193.7286 30 5.81 193.76 4708.762 165.0898 50 6.60 132.095053.512 186.1951 200 10.14 50.72 5390.051 199.0091 500 13.68 27.365696.957 182.425 700 16.14 23.06 5982.621 143.9863 1000 19.65 19.656323.965 161.3212 1500 25.41 16.94 6663.049 176.3982 1900 30.56 16.09

TABLE 12 Rheology testing of 0.3% XG/ 0.05% PHPA/0.02% Pac-LV/2.0%Bentonite t in s t_seg in s Á in 1/s , in Pa f in mPas 493.5 147.75120.003 0.52 172307.23 816.9511 146.2436 0.004 0.80 200685.19 1126.014130.4267 0.005 1.06 211506.25 1471.018 150.7969 0.006 1.28 213158.561788.734 143.9881 0.007 1.43 203720.97 2152.341 179.4121 0.008 1.53191437.36 2459.043 162.1312 0.009 1.60 178226.09 2759.494 137.1994 0.0101.67 166686.52 3114.159 167.3529 1.7 4.37 2567.80 3412.922 140.9674 3.44.70 1381.68 3750.903 154.6025 170 11.31 66.55 4083.364 162.0737 34014.90 43.84 4412.109 165.8419 510 16.45 32.26 4727.639 156.8378 102023.43 22.98 5044.325 148.5117 1900 35.79 18.84

As can be seen from FIG. 10, the low end rheology of both theXG/PHPA/Pac-LV/Bentonite formulations was far superior to that of XG,Pac-LV, GG, Pac-RV or bentonite alone. Indeed, at shear rates of 0.011/s or less, the viscosity of the XG/PHPA/Pac-LV/Bentonite formulationswas about 80000 cp or higher. A formulation comprising XG alone was thenext best performing fluid; however, the viscosity of this fluid wasonly about 6000 cp at a shear rate of 0.01 1/s. FIG. 11 also establishesthat both of the XG/PHPA/Pac-LV/Bentonite formulations maintainedcomparable high end rheology to fluids comprising XG, Pac-LV, GG, PHPA,Pac-RV or bentonite alone. These results confirm that a drilling fluidcomprising XG/PHPA/Pac-LV has ideal properties for controlling fluid andcutting loss when encountering fracture formations during drilling whenit is combined with a particulate component (such as bentonite).

The rheology properties of the drilling fluid comprising the minimalxanthan gum/PHPA/Pac-LV polymers was then compared to the rheologyproperties of commercially available drilling fluids. The results arepresented in Tables 13 to 15 and FIG. 12.

TABLE 13 Rheology testing of Spectrocap-low end rheology t in s t_seg ins Á in 1/s SS, in Pa f in mPas T in ° C. 359.0471 321.1176 0.001 0.005765760.029 22.74735 763.2167 330.1583 0.003 0.015078 5026.417 22.718331064.186 237.9069 0.005 0.024109 4822.108 22.69931 1417.169 201.73220.007 0.029799 4257.065 22.66565 1790.797 182.7294 0.009999 0.0373013730.266 22.64119 2192.218 189.2127 0.03 0.069844 2328.209 22.6272580.758 244.2333 0.049996 0.082225 1644.576 22.6003 2926.804 254.03390.069997 0.089925 1284.696 22.57857 3140.899 134.7686 0.09999 0.1191011190.768 22.57535 3365.716 149.2567 0.3 0.183881 612.8627 22.569553537.485 107.2335 0.5 0.231559 463.1706 22.55654 3738.634 87.74985 0.70.262 374.3164 22.54093 3961.325 101.3151 1 0.300032 300.0535 22.525614165.068 90.91603 3 0.436589 145.5384 22.52 4391.211 103.1255 5 0.528714105.7526 22.51677 4593.303 90.92644 7 0.604227 86.31455 22.508864821.184 104.4857 10 0.696482 69.64965 22.49474

TABLE 14 Rheology testing of CR650-low end rheology t in s t_seg in s Áin 1/s SS, in Pa f in mPas T in ° C. 426.2053 343.7263 0.001 0.0082568256.368 21.93053 745.8175 268.6563 0.003 0.018967 6322.704 21.937131084.03 209.8645 0.005 0.026083 5216.961 21.93178 1500.902 233.38050.007004 0.0325 4642.046 21.91244 1877.807 217.1108 0.009999 0.0411874118.854 21.91386 2277.245 223.1302 0.029999 0.070507 2348.99 21.912452622 233.6873 0.049998 0.081652 1633.663 21.89673 2943.926 220.11480.069998 0.09299 1328.961 21.89074 3161.376 104.9423 0.099995 0.1129171129.017 21.89735 3365.324 93.63118 0.3 0.270882 902.9397 21.905593574.846 88.25933 0.5 0.320683 641.3433 21.90231 3809.622 107.6515 0.70.35745 510.6405 21.89667 4013.664 95.55291 1 0.400091 400.0573 21.891364225.7 92.76638 3 0.565375 188.4725 21.89338 4437.494 88.6554 5 0.672414134.4908 21.90322 4660.515 96.79767 7 0.755427 107.9194 21.905834875.154 96.34115 10 0.8595 85.95019 21.90279

TABLE 15 Rheology testing of Corewell-low end rheology t in s t_seg in sÁ in 1/s SS, in Pa f in mPas T in ° C. 380.4846 312.9846 0.001 0.006416409.692 22.65231 782.1339 310.2679 0.003 0.015354 5118.571 22.638041168.056 291.2845 0.005 0.023642 4728.831 22.6231 1525.263 244.23680.007 0.031059 4437.333 22.60053 2006.515 322.0182 0.01 0.0407484074.909 22.58545 2301.816 213.3316 0.03 0.0782 2606.776 22.574212664.342 229.9184 0.050007 0.094729 1894.447 22.55211 2972.75 193.73590.069998 0.107234 1532.531 22.54219 3239.138 116.2515 0.099994 0.1419361419.277 22.54553 3456.885 109.9146 0.3 0.231692 772.5788 22.536543663.474 92.27832 0.5 0.293 585.9568 22.52516 3913.695 118.5198 0.70.340126 485.7411 22.51779 4114.667 95.44382 1 0.395676 395.704922.51824 4321.568 77.80147 3 0.606389 202.1126 22.51558 4527.99 59.713275 0.747327 149.4723 22.51208 4771.294 79.15696 7 0.851824 121.696122.50255 4992.5 76.44786 10 0.97896 97.89492 22.49294

As can be seen from FIG. 12, the XG/PHPA/Pac-LV and XG/PHPA/Pac-LV/GGformulations demonstrated superior low end rheology compared to threecommercially used drilling fluids. Specifically, at any given low endshear rate, the viscosity of the XG/PHPA/Pac-LV and XG/PHPA/Pac-LV/GGformulations was higher than to commercial products.

Example 2 Field Trials of Drilling Fluid Formulations

Various drilling fluid formulations, some of which are described inExample 1 above, were tested in the field during fluid loss conditions.FIG. 13 shows the results of a field trial of CTroI (0.18% w/w XanthanGum+0.05% w/w PHPA+0.02% w/w Pac-LV+0.02% w/w Guar Gum+0.01% w/w Pac-RV)and CTroIX (0.3% w/w Xanthan Gum+0.02% w/w Pac-LV+0.02% w/w GuarGum+0.01% w/w Pac-RV+1.2% Bentonite+4.8% w/w Fibre) in the Brukungapyrite mine located in South Australia, Australia.

The borehole was initially undergoing a significant fluid loss in therange of 50 to 90 percent of the circulating flow rate. Aftercharacterising the fluid loss using water, two commercial polymer fluidswere circulated in the borehole (Commercial Polymer 1—Pac-RV at 0.2%w/w; Commercial Polymer 2—Pac-RV at 0.3% w/w), resulting in fluid lossreduction. However, when CTroI or CTroIX drilling fluid was subsequentlyused, the fluid loss decreased even further due to the superior shearthinning properties of CTroI and CTroIX. Specifically, the fluid lossdropped to a range of 9 to 15 percent of the circulating flow rate.Furthermore, when CTroIX was used, fluid loss was reduced even further(i.e. to less than 3 to 5 percent of the circulating fluid).

A separate field trial was performed at the Brukunga site duringdrilling of another borehole. The borehole was created using a drillingrig made of downhole motors which are sensitive to drilling fluidscomprising solid particles. The borehole intersected a network offractures at a depth of 125 metres, and therefore suddenly led to acomplete loss of drilling fluid during drilling (see FIG. 14).

In order to remedy this, the CTroIX formulation (modified to alsoinclude 0.05% w/w PHPA) and was injected into the annulus of thedrilling rig and the drilling continued. As shown in FIG. 15, thisresulted in the gradual improvement of fluid return. Once the return wasestablished, the drilling was continued with a minimum amount of fluidloss thereafter (as shown in FIG. 16).

A third field trial was conducted in Victoria, Australia using the samedrilling rig as the second trial referred to above. In this trial, adrilling fluid comprising 0.18% w/w Xanthan Gum+0.05% w/w PHPA+0.02% w/wPac-LV+0.02% w/w Guar Gum+0.01% w/w Pac-RV+4.0% w/w KCl was used todrill through a 135 metre interval of unconsolidated ground. During thedrilling, as the drill bit was exposing new formations, significantfluid loss was noticed as shown in FIG. 17. However, as the drillingfluid invaded into the formations, the fluid effectively sealed theunconsolidated ground, and therefore minimised drilling fluid loss. FIG.18Error! Reference source not found. shows a number of cycles ofexposing new formations, occurrence of significant fluid loss, andsealing the loss zones within a minute as the drilling fluid invadedinto the formation.

As demonstrated herein, a series of experiments has been performed toobtain a blend of polymers which can provide extended shear thinningproperties, i.e. having high viscosities at low shear rates and lowviscosities at high shear rates. A formulation comprising the minimalpolymer components xanthan gum/PHPA/Pac-LV was obtained. Thisformulation was further blended with additional components and tested indrilling field trials. Two blends were tested, CTroI and CTroIX. CTroIwas shown to be a preventative drilling fluid system used whiledrilling. CTroI can seal unconsolidated formations and has resistanceagainst fluid loss. The field trials showed that CTroI can control fluidloss within a few minutes of invasion into very unconsolidated andpermeable formations. While drilling fractured formations, if theaperture of the fracture, and therefore conductivity, will be large,then fluid loss will be high. This is where CTroIX was shown to beuseful. As CTroIX is a remedial solution, it would likely only berequired to be injected into the annulus of the drilling machinery.Indeed, the Brukunga field trials showed that CTroIX is effective incontrolling fluid loss control in broken and fractured formations.

Example 3 Graphite as a Lost Circulation Material

The following modelling experiment was carried out to test theeffectiveness of including graphite (as a lost circulation material) inpreventing drilling fluid loss upon encountering a fracture formation.The experiments used a specifically engineered fluid loss simulatorwhich measured the initial and final permeability of actual formationsby converting flow rates through the sands at set inlet pressures. Also,benchmarking of fluid loss was performed to define the drilling fluidloss through different formation sizes.

The experiments were conducted using a base polymer fluid (0.2% w/wxanthan gum), and a base test drilling fluid (0.18% w/w xanthangum/0.02% w/w Pac-LV/0.02% w/w guar gum/0.05% w/w PHPA) into whichgraphite was added. The graphite had a particle size distribution (D50)of 454 μm using a Malvern Mastersizer 3000 (Malvern Panalytical Ltd,United Kingdom) and SG of 1.9-2.3. The concentrations of graphite testedwere 0, 1, 2, 3, 4, 6 or 10% w/w. The sand sizes used for the testingincluded 6.4 mm, 3.2 mm, 2.0 mm, 1.0 mm, supercut and superfine. Tobenchmark fluid loss, standard bentonite of concentration 12 g/350 ccwas used.

All fluid loss experiments were conducted using a Fluid Loss Simulator(FLS). Model setup is shown FIG. 19. The FLS was built to overcome thelimitations of conventional fluid loss experimentalequipment—Permeability Plugging Apparatus (PPA) and API filter press.Limitations such as inability to visually analyse procedure in PPA, andmeasuring filtrate loss using filter paper in API filter press areovercome. Also, formations are simulated using actual sands rather thantapered discs/slots.

The FLS consists of an inlet pressure cap with a wika sensor (D)connected to an air compressor (C) to supply the required pressure intothe graduated acrylic tube cell body (A) filled with sand to simulateformation as well as fluid (drilling mud). The hose connected to thebottom of the tube is linked to a receiving cylinder (B) at the bottomwhich is connected to a 1 Bar pressure sensor (E) to read the pressureof fluid being received by the cylinder as the fluid flows through theformation. Both pressure sensors are connected to a DAQ and then acomputer with a Data Acquisition software installed. Calibration ofequipment is done after every experiment by pouring back 400 mL of fluidinto the receiving cylinder in 4 steps of 100 mL which is then recordedas pressure per volume. A characteristic calibration plot is shown inFIG. 20. Sands are placed in the tube up to 300 mm and then completelysaturated with water, while test fluids are poured 250 mm from the topof the sand to the 550 mm mark. The bottom valve helps to control flowinto the receiving cylinder.

Benchmarking Fluid Loss: 12 g/350 cc of API Bentonite was mixed for 15minutes until homogenous. Initial sand permeability was tested using theFLS by running water through 6.4 mm, 3.2 mm, 1.0 mm, supercut andsuperfine sands under pressure. Evaluation of Graphite Bridging andSealing Capacity: Base fluids were first mixed for 15 minutes to ensurecomplete hydration of polymer. Graphite was then added to the basefluids and again mixed vigorously before being pushed into the sand. Thefinal permeability of the formation was derived by testing water flowthrough the sand after graphite had been pushed into the sand. Graphiteconcentrations analysed here were 0%, 1%, 3% and 6% with the twodifferent polymer based drilling fluids as indicated above.

Benchmarking Fluid Loss. In order to characterize mud loss, using theFLS, initial formation permeability was first determined and thebentonite was flown through to give final permeability. It was evidentthat a smaller sand size is indicative of a more compact formationresulting in a higher percentage reduction in permeability to about99.8% of its original in superfine sand. This could be as a result of anobvious mud cake formed on the surface of the superfine sand. Thisevident mud filter cake was not observed in the larger sized sand (e.g.6.4 mm) which had a total fluid with a 150 cc/sec flow rate. Theexperiment on the 3.2 mm sand showed an anomaly where final permeabilitywith bentonite flowing through the sand was higher than initial withwater flowing. This was later found to be as a result of different inletpressures with initial 7 psi being almost twice the final—3.48 psi. Itcould therefore be said that keeping the pressure differential constantis significant when determining permeability using the fluid losssimulator.

Evaluation of Graphite Bridging and Sealing Capacity. To evaluate thebridging and sealing capacity of graphite, the fluid loss simulator usedthe 6.4 mm sand to simulate severe fluid loss in highly unconsolidatedformations. Two experiments each were performed with 1%, 3% and 6%graphite concentrations. During every experiment, sand initialpermeability experiments were performed with water at differentpressures. These initial permeability results as well as the finalpermeability results after treating fluid loss with differentconcentrations of graphite and different base fluids are shown in Table16.

TABLE 16 Summary of Graphite Performance Base Fluid XG XG + Pac-LV +GG + PHPA Initial 0% Graphite 2713.992 Permeability 1% Graphite 5862.9661200.098 3% Graphite 6902.786 2713 6% Graphite 2708.123 2836.294 Final0% Graphite 2109.162 Permeability 1% Graphite 6.707 2.427 3% Graphite2.06 0.2075 6% Graphite 0.2091 0.01104

Data in Table 17 and plotted in FIG. 20 shows the percentage reductionin permeability as a result of increasing the concentration of graphitein the drilling fluids tested.

TABLE 17 Permeability Reduction of Graphite in 6.4 mm Sand Base Fluid XGXG + Pac-LV + GG + PHPA 1% Graphite 99.81% 99.81% 3% Graphite 99.85%99.99% 6% Graphite 99.99% 100.01% 

It can be seen that by adding 1% graphite to base fluid, the highlyunconsolidated 6.4 mm sand, which had earlier experienced total fluidloss, had its permeability reduced to upwards of 99.8% with both basefluids, and completely plugging the formation reducing permeability at6% graphite concentration. Visual analysis after treating with graphiteshowed that graphite particles invaded the pore spaces, bridging thepore throat and reducing the permeability.

1. A borehole drilling fluid comprising: (i) about 0.1% to about 0.5%w/w xanthan gum; (ii) about 0.02% to about 0.1% w/w low molecular weightpartially-hydrolysed polyacrylamide (PHPA), wherein the PHPA has amolecular weight in the range of about 10000 Da to 10000000 Da; and(iii) about 0.02% to about 0.1% w/w low viscosity polyanionic cellulose(Pac-LV), wherein the drilling fluid exhibits an increase in viscosityunder low shear rates, and exhibits a decrease in viscosity under highshear rates.
 2. The borehole drilling fluid of claim 1, wherein: (i)when the shear rate of the drilling fluid is less than about 0.01 1/s,the viscosity of the drilling fluid is about 10,000 cp or higher asmeasured at about 23° C. to about 25° C.; or (ii) when the shear rate ofthe drilling fluid is about 0.01 1/s, the viscosity of the drillingfluid is about 6,100 cp or higher as measured at about 23° C. to about25° C.
 3. (canceled)
 4. The borehole drilling fluid of claim 1, whereinwhen the shear rate of the drilling fluid is about 1000 1/s or more, theviscosity of the drilling fluid is about 12 cp or lower as measured atabout 23° C. to about 25° C.
 5. The borehole drilling fluid of claim 1,wherein the drilling fluid comprises about 0.18% w/w xanthan gum, about0.05% w/w PHPA, and about 0.02% w/w Pac-LV.
 6. The borehole drillingfluid of claim 1, wherein the drilling fluid further comprises one ormore of guar gum, regular viscosity polyanionic cellulose (Pac-RV), andpotassium chloride.
 7. The borehole drilling fluid of claim 5, whereinthe drilling fluid comprises up to about 0.1% w/w guar gum, up to about0.1% w/w Pac-RV, and/or up to about 8.0% w/w potassium chloride.
 8. Theborehole drilling fluid of claim 5, wherein the drilling fluid comprisesabout 0.02% w/w guar gum, about 0.01% w/w Pac-RV, and/or about 4%potassium chloride.
 9. The borehole drilling fluid of claim 5, whereinthe drilling fluid comprises: (i) about 0.18% w/w xanthan gum, about0.05% w/w PHPA, about 0.02% w/w Pac-LV, and about 0.02% guar gum; (ii)about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, about 0.02% w/wPac-LV, about 0.02% w/w guar gum, and about 0.01% w/w Pac-RV; or (iii)about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, about 0.02% w/wPac-LV, about 0.02% w/w guar gum, about 0.01% w/w Pac-RV, and about 4.0%w/w potassium chloride. 10-17. (canceled)
 18. The borehole drillingfluid of claim 1, wherein the drilling fluid is substantially free ofsolid particles.
 19. The borehole drilling fluid of claim 1, wherein thedrilling fluid includes solid particles.
 20. The borehole drilling fluidof claim 11, wherein the drilling fluid comprises one or more ofbentonite, fibre, and graphite.
 21. The borehole drilling fluid of claim11, wherein the drilling fluid comprises about 0.01% to about 2.0% w/wbentonite, up to about 5.0% w/w fibre, and/or about 1.0% to about 10%w/w graphite.
 22. The borehole drilling fluid of claim 11, wherein thedrilling fluid comprises about 1.2% w/w bentonite, about 4.8% w/w fibre,and/or about 6.0% w/w graphite.
 23. The borehole drilling fluid of claim11, wherein the drilling fluid comprises: (i) about 0.3% w/w xanthangum, about 0.05% w/w PHPA, about 0.02% w/w Pac-LV, and about 1.2%bentonite; (ii) about 0.3% w/w xanthan gum, about 0.05% w/w PHPA, about0.02% w/w Pac-LV, about 0.02% w/w guar gum, about 0.01% w/w Pac-RV,about 1.2% w/w bentonite, and about 4.8% w/w fibre; or (iii) about 0.3%w/w xanthan gum, about 0.05% w/w PHPA, about 0.05% w/w Pac-LV, about0.02% w/w guar gum, about 1.2% bentonite, and about 6.0% w/w graphite.24-31. (canceled)
 32. A method of reducing borehole drilling fluid lossand cutting loss during borehole drilling, the method comprisingconducting the borehole drilling using a borehole drilling fluidcomprising: (i) about 0.1% to about 0.5% w/w xanthan gum; (ii) about0.02% to about 0.1% w/w low molecular weight partially-hydrolysedpolyacrylamide (PHPA), wherein the PHPA has a molecular weight in therange of about 10000 Da to 10000000 Da; and (iii) about 0.02% to about0.1% w/w low viscosity polyanionic cellulose (Pac-LV), wherein thedrilling fluid exhibits an increase in viscosity under low shear rates,and exhibits a decrease in viscosity under high shear rates.
 33. Themethod of claim 15, wherein: (i) when the shear rate of the drillingfluid is less than about 0.01 1/s, the viscosity of the drilling fluidis about 10,000 cp or higher as measured at about 23° C. to about 25°C.; or (ii) when the shear rate of the drilling fluid is about 0.01 1/s,the viscosity of the drilling fluid is about 6,100 cp or higher asmeasured at about 23° C. to about 25° C.; and/or (iii) when the shearrate of the drilling fluid is about 1000 1/s or more, the viscosity ofthe drilling fluid is about 12 cp or lower as measured at about 23° C.to about 25° C. 34-35. (canceled)
 36. The method of claim 15, whereinthe drilling fluid comprises: (i) about 0.18% w/w xanthan gum, about0.05% w/w PHPA, and about 0.02% w/w Pac-LV; (ii) about 0.18% w/w xanthangum, about 0.05% w/w PHPA, about 0.02% w/w Pac-LV, and about 0.02% guargum (iii) about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, about 0.02%w/w Pac-LV, about 0.02% w/w guar gum, and about 0.01% w/w Pac-RV; or(iv) about 0.18% w/w xanthan gum, about 0.05% w/w PHPA, about 0.02% w/wPac-LV, about 0.02% w/w guar gum, about 0.01% w/w Pac-RV, and about 4.0%w/w potassium chloride. 37-48. (canceled)
 49. The method of claim 15,wherein the drilling fluid is substantially free of solid particles. 50.The method of claim 15, wherein the drilling fluid includes solidparticles. 51-53. (canceled)
 54. The method of claim 19, wherein thedrilling fluid comprises: (i) about 0.3% w/w xanthan gum, about 0.05%w/w PHPA, about 0.02% w/w Pac-LV, and about 1.2% bentonite; (ii) about0.3% w/w xanthan gum, about 0.05% w/w PHPA, about 0.02% w/w Pac-LV,about 0.02% w/w guar gum, about 0.01% w/w Pac-RV, about 1.2% w/wbentonite, and about 4.8% w/w fibre; or (iii) about 0.3% w/w xanthangum, about 0.05% w/w PHPA, about 0.05% w/w Pac-LV, about 0.02% w/w guargum, about 1.2% bentonite, and about 6.0% w/w graphite. 55-62.(canceled)